MICROSCOPE, USES OF THE. 



MICROSCOPE, USES OF THE. 



be increased by pasunir from the object-glasses of lower power to 

 those of higher power ; out, nevertheless, for seeing with convenience, 

 and especially for drawing, the use of a powerful eye-glass is often 

 not without advantage. As long u the magnifying power can be 

 increased by means of an object-glass, recourse should never be had 

 to the eye-glass, since both the light and the shsrpness of outline of 

 the image are necessarily diminished by the use of a powerful eye- 

 glass, which is not the ease in using a more powerful object-glass. 



In some cases, it is a good plan to shade with the left hand, the 

 eye which looks into the microscope. When an object is thin enough 

 to be seen with transmitted light, it is first illuminated with light 

 transmitted directly, and is examined with different, and gradually 

 increasing, magnifying pqwers ; should any details of the image remain 

 undefined, obliquely transmitted light is used, which U insinuated 

 into all the different corners of the object In some microscopes this 

 is attained by turning the stage round its axis ; where this arrange- 

 ment is wanting, the position of the object must be changed by moving 

 it with the hand. Lines always stand out most clearly when oblique 

 light falls upon them at a right angle : where, therefore, a line is sus- 

 pected to exist, or is only dimly seen, particular attention must be 

 paid to this circumstance. In submitting objects to incident light, the 

 same rule generally holds good, and particular care must be taken, by 

 turning either the stage or the object itself, to concentrate the light 

 in all possible directions upon the object. Object-glasses of very 

 high power cannot be used with incident light, inasmuch as the 

 shortness of their focal length prevents the light from falling on the 

 object ; in this case recourse must be had to less power object-glasses, 

 and more powerful eye-glasses. As a general rule, low-magnifying 



Objects are frequently examined by polarised light. In order to 

 effect this an instrument called a polarucope is employed. That most 

 frequently used is the arrangement proposed by Mr. Nichol. It 

 consists of two prisms of Iceland spar, one of which is fitted beneath 

 the stage, the other is attached to the eye-piece. Tourmaline is also 

 used for the same purpose. Large crystals of iodide of quinine have 

 also been shown by Dr. Herapath to be applicable in polarising light 

 for the microscope. ('Quarterly Journal of Microscopical Science,' 

 vol. ii.) By the use of polarised light objects frequently exhibit 

 their structure in a more perfect manner. Various objects, especially 

 crystals of a spherical or oval form, exhibit a beautiful variety of 

 colour in this way. In some cases it may be made the means of 

 testing the nature of an object 



In most instances, objects are examined under water : it is but 

 seldom, as, for instance, in examining pollen or spores, that it is neces- 

 sary to observe them in different media, and also when dry. In the 

 case of incident light, water often operates injuriously, especially when 

 the object is not quite covered by it : it is therefore advisable, for 

 certain particular objects, as, for instance, the embryoes of grasses, to 

 observe them first without water, and afterwards under water; by 

 placing them under a cover, and adding water with a camel's-hair 

 brush, the object is generally sufficiently and fully immersed. When 

 low-magnifying powers are used, it is not necessary that the objects 

 should be placed under a glass cover, in fact in many cases where it 

 is wished to have the power of turning the object round, or when it is 

 thought that the object may be improved by any additional cutting 

 or preparation, it is very advantageous not to cover it ; when object- 

 glasses of very high power are used, the focal distance is so short, that 

 in order to prevent striking the lens against the object, or dipping it 

 in the fluid upon the object plate, it is necessary to make use of glass 

 covers. When these are used, the fluid in which the object lies fre- 

 quently becomes lessened by evaporation during the observation, in 

 which case a fresh drop is added at the edge of the glass cover by 

 means of a glass rod, or a clean camel's-hair brush, which may be used 

 when it is wished to add a solution of iodine, or of chloride of zinc and 

 iodine, to objects which are already immersed in water. 



When any chemical re-agents are used, whether iodine, caustic 

 potash, or an acid, the object should always be covered with a thin plate 

 of glass ; in using volatile acids, such as nitric acid and hydrohloric 

 acid, too much care cannot be taken. The vapour of sulphuretted 

 hydrogen has a very injurious effect upon flint glass, which is used by 

 some opticians for the under side of the object-glass. 



When the microscope is in daily use, it is a good plan to keep it 

 under a high bell-glass, or an ornament shade. 



The greatest cleanliness and accuracy are indispensable for micros- 

 copical investigations : it must be laid down a* a rule always to use 

 the cleanest water, in the cleanest vessels, for moistening the slides. 

 Even with this precaution it is impossible entirely to protect the 

 object from becoming soiled with particles of dust Extraneous 

 thingn of this kind will not easily deceive a practised observer ; a 

 beginner however may be easily misled by them. Water which has 

 been left standing should never be used, since it too frequently con- 

 tains the inferior sorts of animals and plants; and when different 

 objects are examined one after another, fresh water should be taken 

 for every new object, in order that no particles of the objects which 

 have been previously examined may be mixed with the water upon 

 the slide. Many errors may be traced to a neglect of small precautions 

 of this sort 



In order to be able to recognise extraneous objects as such, it is 



advisable to gain an acquaintance with those things which, notwith- 

 standing all precautions, cannot always be avoided. To this class of 

 things belong 1st, Air-bubbles, which, with transmitted light, gener- 

 ally appear in the form of circles of larger or smaller diameter, with 

 a dark, black-looking rim : with incident light, on the contrary, their 

 rim appears of a white colour. When the object U under a glass 

 cover and in contact with it, the larger air-bubbles frequently assume 

 a very irregular shape ; the above-mentioned optical fact is generally 

 however by far the best proof of the presence of air, and by it the 

 presence of air may be detected both in and between the cells of 

 plants. 2nd, Colourless or coloured fibres of paper, or of linen, 

 woollen, or silk-textures, left behind upon the object-glasses, from the 

 cloths with which they have been cleaned, and also the hairs which 

 have been detached from the brush. 3rd, Granular particles of dust 

 of irregular shape, which are frequently coloured, and are probably 

 produced by the decay of organised bodies. If it is wished to examine 

 plants, or parts of plants, which grow either in or upon the earth, 

 or in water, great attention must be paid to the many organised bodies 

 which are likely to be met with : pains must be taken by careful obser- 

 vation to become acquainted with the lower forms of animals and 

 plants: it is necessary, for instance, to be able to distinguish the 

 common forms of Infiuoria, both those which are provided with 

 siliceous coatings, and those that are not ; also with the yeast plant, 

 the different forms of mould, the Oirillaioria, and such like things, in 

 order to be able to separate them from the particular object under 

 consideration. 



The epithelial cells of the mucous membrane of the mouth are also 

 objects which may deceive the observer. They occur when the brush 

 is drawn through the mouth previously to bringing an object upon 

 the object-plate. It is advisable never to pass the brush through the 

 mouth. When in cutting small objects, the latter are held between 

 the thumb and forefinger, or upon the forefinger alone, it often happens 

 that small fragments of the skin of the finger are cut off at the same 

 time. The observer must learn to distinguish these fragments, as well 

 as the small pieces of cork which he will meet with in sections made 

 between that substance. 



Appearances of motion, either usual or accidental, may also give rise 

 to mistakes, and these must therefore be learnt. Molecular motion is 

 peculiar to all very small bodies, contained in a thin fluid medium ; it 

 consists of a somewhat trembling motion of these small bodies ; it is 

 frequently seen in the interior of pollen grains; it may be observed 

 still better in certain fluids, for instance milk, when a small quantity 

 is mixed with water, and placed under the microscope, with a magni- 

 fying power of from 200 to 400 diameters. When acquaintance is 

 once made with this phenomenon no further deception can be caused 

 by it The same result follows from accidental current* upon the 

 object-plate, which may take place either by evaporation or by the 

 mingling of two fluids of unequal specific gravity, or by the dissolving 

 of any salt existing in the fluid. 



Observations are made less frequently with reflected than with 

 transmitted light, but since the latter can only be used for very thin 

 objects, the principal point to be attended to in dealing with opaque 

 objects, is to make such an arrangement of them, as to enable the 

 observer clearly to make out their details. The manner in which the 

 object is divided must be regulated and altered according to the 

 nature of the object itself, and the information which it is wished, by 

 the help of the microscope, to obtain respecting it Firm homoge- 

 neous textures, suoh as wood, must be treated quite differently from 

 delicate objects composed of different organs, such as buds aud 

 blossoms ; in the case of wood it is sufficient to take as thin a slice as 

 possible, cut in a certain fixed direction ; in the case of buds and 

 blossoms, attention must be paid not only to the direction, but also, 

 particularly, to the point at which the section is made ; it is necessary 

 to exhibit an accurate longitudinal section through the middle of the 

 whole bud or blossom, and an equally accurate transverse section made 

 at different heights, in order to ascertain the arrangement of the 

 organs with respect to one another; moreover, the different parts of 

 the organs must be separated and examined by themselves ; in cases 

 like this, and especially in inquiries connected with the development 

 of plants, a dissecting microscope is necessary. The same remarks 

 apply to hard and soft animal tissues. 



Succulent or spongy tissues have generally large cells ; it is not 

 necessary therefore to have thin sections of such tissues, which are 

 always difficult to make. Delicate animal tissues may advantageously 

 be placed in spirit or pyroligneous acid for some days, provided it is 

 not necessary that the tissues should be examined whilst fresh ; but 

 there is little advantage to be derived from treating botanical objects 

 in that manner. It is a good plan however, in many cases, to saturate 

 delicate portions of animals and vegetables with thick gum-mucilage, 

 and to let them dry slowly in the air. 



In dissecting, different methods must be adopted, according to the 

 magnitude of the different objects ; objects of large size may be held 

 with the left band, or with the thumb and forefinger of that hand ; 

 very small or very thin objects, such as the stems of mosses, thin 

 twigs and roots, leaves, small seeds, and such-like things, may be 

 placed between two pieces of cork, and thin slices of the object cut 

 by means of a sharp knife or razor. 



Observations are sometimes disagreeably impeded by the presence 



