MICROSCOPE. 



659 



cither of cross- wires moved by an extremely delicate 

 adjustment; of minute bodies whose dimensions 

 have been ascertained, such as the globules of the 

 blood, certain small seeds, &c. ; or of minute parallel 

 lines drawn at regular distances, on slips of glass, 

 with the point of a diamond. Now, it is obvious, 

 that if with any power of the microscope, we bring 

 one of these measures under observation, in con- 

 nection with a minute object whose dimensions are 

 sought, we can establish a comparison more or less 

 correct that will solve the inquiry. Sir David 

 Brewster remarks, that "a good micrometer for 

 single microscopes is still, however, a desideratum ;" 

 he also observes that " an excellent method of 

 measuring microscopic objects, is to project the 

 image of the object against a divided scale, at a 

 given distance from the eye." Now, this method 

 is precisely the one adopted by Dr Hooke ; and as 

 it is the most simple, arid for ordinary purposes 

 the most satisfactory means that can be employed, 

 we shall briefly describe and illustrate its applica- 

 tion. Any plain scale that is accurately divided 

 will answer the purpose ; and it is only necessary 

 to lay it on the table at the foot of the microscope, 

 which should be at such height, that the eye, when 

 making observation, may be just ten inches from 

 the scale. Now, whilst looking with one eye into 

 the microscope, observe with the other the divisions 

 on the scale, and refer the magnified object to those 

 divisions, by which means a diametric measurement 

 of the enlarged image is readily obtained. A per- 

 son who has never used this method will scarcely 

 believe with what readiness and accuracy, after a 

 few trials, the measure maybe determined. Then 

 to ascertain the actual dimensions of the object, 

 nothing more is required than the magnifying power 

 employed. For instance : we observe that a cer- 

 tain object seen through the microscope measures 

 five-tenths of an inch on the scale, and the magni- 

 fying power is 200 linear ; divide five-tenths there- 

 fore by 200, and you obtain one four-hundredth 

 part of an inch, as the actual diameter of the object, 

 The magnifying power itself may of course be 

 readily determined by bringing an object under re- 

 view whose dimensions are known, and projecting 

 its magnified image on the scale. Thus, if an 

 object one-tenth of an inch in length be projected 

 over two inches of the scale, the microscope has a 

 Irnear power of twenty. Those who desire fuller 

 information respecting the recent improvements in 

 the microscope and its appendages, should consult 

 Goriny's Microscopic Cabinet, Goring and Prit- 

 chird's Microyraphia, and Brewster's Treatise on 

 the Microscope. 



THE PREPARATION AND EXAMINATION OF OB- 

 JECTS. A satisfactory development of the natureand 

 structure of microscopic objects, depends altogether 

 on a judicious preparation of the objects themselves, 

 and the application of suitable light and magnifying 

 power. As much disappointment is sometimes felt 

 by persons who pursue their observations without 

 due consideration of these important points.it may 

 not be amiss to lay down a few general rules with 

 reference to them. All objects are either opaque 

 or transparent ; and are to be viewed either by rays 

 reflected from their surfaces or by light transmitted 

 through them. Now, as it is inconvenient with any 

 but low powers, and altogether impracticable with 

 very deep ones, to condense a sufficiency of light 

 on the surface of an object, to render observation 

 pleasant and satisfactory.it is generally recommend- 

 ed to reduce opaque objects to transparency, by 



cutting them into very thin sections. There is a 

 variety of objects, however, that cannot be treated 

 in this way without destroying them, and which 

 must consequently be viewed by reflected light ; 

 add to this, that the structure of few transparent 

 objects can be thoroughly ascertained until they are 

 observed both by transmitted and reflected light. 



When an object is selected for examination, we 

 must first consider in what way it can be treated. 

 If it be opaque, and of such a nature that a thin 

 section cannot be obtained, or if obtained, would 

 give no correct idea of the general structure, it is 

 best to illuminate it with reflected light, and view 

 it under a low power; afterwards detached por- 

 tions can be observed with a higher magnifier, and 

 the examination of these may perhaps be pursued 

 farther by a judicious subdivision that shall render 

 them sufficiently transparent to admit of light being 

 transmitted, and much deeper magnifiers applied. 

 If the object be naturally transparent, it is advis- 

 able in the first place to view it as if opaque, which 

 is easily done by laying it on a stage of opposite 

 colour to its own, and condensing a strong light 

 upon the surface. The observer thus obtains a 

 knowledge of the superficial structure; and may 

 then proceed in his examination by transmitted 

 light, gradually increasing the magnifying power. 

 The surfaces of opaque objects, and the degree of 

 transparency in diaphanous ones, vary very consider- 

 ably; and hence the supply of light must be limited 

 or increased accordingly. Some opaque objects 

 have such an absorbing surface, that the strongest 

 light is barely sufficient to answer our purpose ; 

 others present all the variety of colours, each re- 

 quiring a peculiar modification of the light ; a third 

 class have surfaces so highly burnished that a strong 

 illumination dazzles and perplexes the eye ; a fourth 

 class exhibit such a confusion of prismatic reflec- 

 tions, that the light requires not only to be duly 

 modified, but also to be cast upon them at a pro- 

 per angle, otherwise, we see a gorgeous nothing. 

 And with respect to transparent objects, some are 

 slightly inclined to opacity, and require a very 

 powerful transmitted light to develope them ; 

 others are so exceedingly transparent that it is 

 impossible to examine them, except with a faint 

 illumination ; and an almost infinite variety lies 

 between these two extremes, every individual dif- 

 ference requiring a modification of light peculiar to 

 itself, and which can be ascertained only by trial. 



In the examination of objects, there are two very 

 important points to be attended to with reference 

 to the magnifying powers employed : First, to 

 make the observation with no deeper power than 

 is absolutely necessary to develope the structure 

 satisfactorily ; and, secondly, to determine whether 

 it is penetrating or defining power that is needed. 

 The opticians of a century past fell into a great 

 error in the construction of the microscope ; they 

 considered that the merits of the instrument lay 

 in a vast enlargement of the field, and a very deep 

 objective power: the consequence was, that they 

 produced enormous images totally destitute of de- 

 finition, and exhibited under an attenuated light 

 barely sufficient to make darkness visible. Micro- 

 graphers, taking their cue from the opticians, (or 

 the opticians from them, we know not which) de- 

 lighted in the use of deep powers; and so long as 

 they could magnify the object to wonderful ex- 

 tent, they appear to have had small concern about 

 the mistiness and indecision that interfered with 

 their observations. No person who is conversant 

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