813 



MICROSCOPE, USES OF THE. 



MICROSCOPE, USES OF THE. 



814 



Although dealing with the disposition of large masses of matter, of 

 which the earth's surface is composed, important knowledge is obtained 

 by the geologist by examining minute portions of them with the aid 

 of the microscope. It ia by the aid of this instrument alone that the 

 question of the manner in which an estensive series of rocks has been 

 deposited can be determined. Many rocks which present a homoge- 

 neous surface, when sections are made of them and placed under the 

 microscope, are found to consist of the remains of the minutest forms 

 of organic beings. [DiATOMACEJ: ; INFUSORIA.] Whilst, on the other 

 hand, rocks which, like the Oolites, present to the naked eye the 

 appearance of being composed of various /orms of organic beings, 

 on close examination with the microscope are found to present appear- 

 ances due to purely physical causes. Each of the little egg-like bodies 

 seen in these rocks is found to contain a particle of sand surrounded 

 by carbonate of lime. [MAGNESIAN LIMESTONE ; OOLITIC SYSTEM.] 

 It frequently happens that the fossilised parts of the higher animals 

 which are left in the solid rock are too small or shapeless to indicate 

 the characters of the family to which they belonged ; but by the aid 

 of the microscope this question can be determined in multitudes of 

 instances. It has been found by the recent researches of Dr. Carpenter 

 and others, that the microscopic characters afforded by the structure 

 of shells are frequently so distinct as to afford the means of distin- 

 guishing between allied species of Molluaca. This is a subject of the 

 highest interest to the geologist ; for of all the remains of animal life 

 those of the Molluaca, from the hard nature of their shells, are those 

 which are furnished to him in greatest abundance for distinguishing 

 the character of different strata, and determining their relative age. 

 Amongst the vertebrate animals, next to the teeth, the bones are the 

 parts which are most frequently left in rocks. When the form of the 

 bone is no guide, its microscopic structure will determine the family 

 to which it belongs. The forms presented by the lacuna;, or bone-cells 

 [BONE], which are only visible by the aid of the microscope, are found 

 to differ so much in fish, reptiles, birds, and mammals, as to afford the 

 means of distinguishing each class. The teeth are only modified bone, 

 and although they present most frequently a very definite external 

 form, and are usually better preserved than any other part of an 

 animal body, they afford still more definite characters by the modifi- 

 cations of the internal portions of their structure. It is thus that the 

 internal structure of the tooth presents a better character than any 

 other part of its structure in the gigantic extinct frog, the Laby- 

 rinthodon. [AMPHIBIA.] 



Among the earlier fishes which inhabited the seas of the earth were 

 principally those which have a cartilaginous skeleton ; but with this 

 perishable skeleton they were endowed with hard scales, which have 

 resisted all decomposing agencies, and these scales, frequently scattered, 

 are the only record of their existence. From microscopic examination 

 of the structure of these scales, Professor Williamspn has shown that 

 the species of those fishes can be detected. 



Numerous are the forms of Zoophyta, Polyzoa, and Echinodermata 

 that have left remains in the strata of the earth which the naked eye 

 can by no means distinguish, but whose differences are immediately 

 revealed when sections of their fossilised remains are placed under 

 the microscope. Examples of these will be found in the second 

 volume of Professor Quekett's ' Lectures on Histology.' 



We pass from this glance at the use of the microscope to the 

 chemist and geologist, to point out its value in researches connected 

 with living organised beings. We begin with plants. It is in the 

 vegetable structure that the ultimate cells, of which all organised 

 beings are composed, are most easily discerned. The earliest micro- 

 icopic observers were aware of this. It was left however for Schleiden 

 to perceive the full significance of this fact in 1838, when he drew 

 attention to the cytoblast as the source of the growth and life of the 

 celL [CELLS.] He was followed by Schwann, who pointed out that 

 the cell was of equal importance in the animal as the vegetable king- 

 dom. The ultimate cell of the plant or animal is only visible by the 

 aid of the microscope ; hence whatever importance is attached to the 

 discovery of cells and the formation of tissues by their agency is due 

 to the use of the microscope. 



The unceasing labours of microscopic observers during the last 

 quarter of a century have developed a new branch of science called 

 histology [HISTOLOOT], which embraces a knowledge of the ultimate 

 structure of the tissues of which all parts of plants and animals are 

 composed. 



One of the most interesting facts connected with the history of the 

 microscope is the discovery of the existence of minute animals and 

 plants, inhabiting more especially water, and which are perfectly 

 invisible to the naked eye. The earliest observations on these minute 

 beings were made by Leeuwenhoek in the middle of the 17th century; 

 but it was left for Ehrenberg, during the present century, to make 

 known to its full extent the immense variety of forms assumed by 

 these microscopic beings. [DIATOMACE.E ; DESMIDIE.E; INFUSORIA.] 

 Since the publication of the ' Infusionsthierchen,' these minute organ- 

 isms have been a source of increasing interest to microscopic observers. 

 Representing the entire animal or plant in its simplest form, the 

 observations of their growth, structure, and functions have thrown 

 light on some of the most complicated problems of animal and 

 vegetable physiology. Small as they are, they make up for want of 

 nize by the prodigious numbers in which they occur, and the important 



functions they seem to perform in the universe. The unveiling this 

 world of life is entirely due to the agency of the microscope. 



It is not however alone in revealing the minute structure of plants 

 and animals, as the existence of organic beings of minute size, that 

 the naturalist and physiologist are indebted to the microscope. By 

 its aid they have been enabled to investigate the functions performed 

 by cells. If the tissues of plants and animals are built up of cells it 

 was a natural inference that these cells performed an important part 

 in the functions of these tissues. It was soon found that the animal 

 and plant increased in size by the multiplication of these cells ; that 

 the tissues were renewed by their agency; that the function of secretion 

 was performed by them ; in short, that it was by the aggregate of their 

 functions that plants and animals lived. Hence the cell theory, or cell 

 doctrine, has been to the explanation of the phenomena of living 

 beings what the law of gravitation was to the phenomena of the 

 physical universe. Till the application of the microscope the myste- 

 rious function by which new beings in plants and animals were 

 produced was only imperfectly apprehended ; but since its extended 

 employment the laws which regulate this process have become per- 

 fectly obvious, and but little remains to complete our knowledge of 

 the subject. [REPRODUCTION IN PLANTS AND ANIMALS.] 



Having pointed out the objects of the use of the microscope, we 

 now proceed to give some general directions for examining them. In 

 giving these directions we shall follow Dr. Schacht, whose work on the 

 microscope has been translated into English by Mr. Currey. 



One of the principal requisites for microscopical investigation, 

 besides a good instrument, is a proper supply of light. When the 

 position and nature of the apartment can be selected at pleasure, a 

 room should be chosen having windows facing the west or the north, 

 or, what is better, a room with windows towards both those quarters of 

 the heavens. The windows must be as high as possible, since the 

 light received from the horizon is the most favourable ; light reflected 

 from a white wall, or the light of white clouds, is often very 

 advantageous. The light of scudding clouds fatigues the eye by the 

 rapid change in the intensity of the light, besides rendering necessary 

 a continual change in the position of the mirror. No ordinary observa- 

 tion is possible in direct sunlight ; this light is, in the first place, far 

 too dazzling for the eye to bear ; and, in the second place, it causes 

 appearances which give rise to the grossest deceptions. In working 

 with the microscope in the forenoon and in the middle of the day, a 

 room lying to the east or to the south must therefore be avoided : by 

 means of white blinds, or curtains, the inconvenience may, to a certain 

 extent, be avoided. 



Many objects are seen very beautifully by lamp-light, but this light 

 is far more glaring than daylight. When the fight is made to pass 

 through blue glass before reaching the mirror, it bears a greater 

 resemblance to daylight, and is pleasanter to the eye. A piece of 

 white ground-glass, fastened in a wooden frame, and placed before the 

 lamp, will have the same effect. By regulating the light of the lamp 

 in this manner, objects already prepared may be shown very well by 

 night, but it is hardly possible to make fine preparations with such 

 an illumination ; for exact observation, therefore, the day-time only 

 must be selected. In order to intercept the light of the horizon by 

 means of the mirror, the latter is placed at least three feet from the 

 window, the microscope is turned with the mirror towards the light, 

 and the whole instrument, but especially the mirror, is placed in 

 different positions whilst the observer looks through the eye-glass ; 

 the light is, in fact, sought after : when the field of view appears 

 clearest and brightest, the object which is to be observed is pushed 

 under the microscope. 



When it is wished to examine opaque objects with incident or 

 reflected light, the microscope may often be advantageously brought 

 nearer to the window. Since for this kind of illumination a much 

 larger quantity of light is necessary, direct sunlight is sometimes 

 desirab'e; in the absence of this, the condensing lens is used, by 

 means of which the greatest possible quantity of light is concentrated 

 upon the object. In this kind of illumination, the access of light 

 from below, which would interfere with the observation, is prevented 

 by closing the diaphragm. For objects which are altogether opaque, a 

 background which is white, but not glittering, is often advantageous. 



The table at which microscopical observations are undertaken must 

 be sufficiently large, and very firm ; it must be so arranged that all 

 the apparatus which is ever wanted shall be at hand. Much time is 

 spared by attention to this, and in microscopical investigations time 

 passes only too quickly ; moreover, in a very confined space it is 

 impossible to make effectual preparations with the simple microscope. 

 Every object intended for investigation should be examined in the 

 first instance with a low magnifying power, since by that means a far 

 larger portion of the object is seen, and thus a better impression with 

 regard to the whole is obtained. Should the light be too strong, the 

 plane mirror may be used instead of the concave one. When the 

 observer has gained as much information as he can with the low- 

 magnifying power, for instance, one of 50 diameters, or, in some 

 cases, even a less magnifying power, the object-glass is changed for a 

 more powerful one. When the most powerful object-glass has been 

 used, and a still stronger magnifying power is found desirable, then a 

 stronger eye-glass is taken. As a general rule, the eye-glass of lowest 

 power should be used, and, if necessary, the magnifying power should 



