MICROSCOPE. 



227 



fowpound aberration, it would be preferable to make the radii as 

 Micro- 2.2 to 1, and then reduce the dispersive power of the 

 *">{*' oil of cassia by oil of olives, or any other less disper- 

 '"""V"''' si ve oil, till the correction of colour is complete. If the 

 oil of sweet fennel seeds is used, the radius of the an- 

 terior should be to that of the posterior surface, as 0.8 

 to 1." 



VII. On the Magnifying Porter of Compound 3/i- 

 crotcopei. 



On th When the microscope consists of two lenses, or 



"tfy'og of one speculum and one lens, the object is magni- 

 P "" of Bed from two causes ; Jirtt, from the enlargement 

 ' of the image produced by the object-lens or specu- 

 HHS> lum, which is always equal to the quotient, arising 



from dividing the distance of the image from the ob- 

 ject-lens or speculum, by the distance of the object 

 from the same ; and, trconrlly, from the effect produced 

 bv the eye-glass, which is afways equal to the quotient, 

 arising from dividing the distance at which the eye 

 sees distinctly by the focal length of the eye-glass 



Met of these two <j ill therefore be the 



.ifying power of the microscope. Hence culling 



J&t focal length of the eye-gla, I) the distance of 



with two from the object-glass, d the distance of the 



flam*. imap e at which the eye sees ob- 



have the magnifying power or 

 M, by the following formula, 



M- - 



with three If a third, or amplifying glass is added, as in the 



(lanes. ctmrn,r compound microscope, for the purpose of en- 



;ig the field, the magnifying power of the in-tru- 



ment is diminished, and may be found by multiplying 



its magnifying power without the amplifying-glaas, a* 



given by the above formula, by the fraction ; 9 be- 

 ing the focal length of the amplifying lens and 

 L = <t J, J being the distance of the 



first and second glasses, and if the distance of the first 

 and third glasses. Hence we obtain the following 

 general rule. Divide the difference between the dis- 

 tance of the two firt lenses, or those next the object, and 

 the focal distance of the second or .impIifying-glaM, by 

 the focal distance of the Mcond flaw, and the quotient 

 will be a -tance between 



the two rirrt lenses, and divide it by the difference be- 

 twei '.ince, and the focnl distance of the se- 



cond gla*- :hii quotient subtract the c-rcew of 



the distance between the first and third glass above the 

 focal length of the third glass, and divide the remain- 

 der by the focal distance of the third glass, or that next 

 the eye, ami a tecond number will be obtained. Mul- 

 tiply together the first and second numbers, and the 

 magnifying power of the object-glass as found from 

 the rule in p. 221. col. 1, and the product will be the 

 magnifying power of the compound microscope. 

 Ntwmode The preceding rules for calculating the magnr 

 of repce- powers of microscope*, are founded on the 

 tentiof the w hich have been adopted by all optical authors ; but 

 H lf J | B it w in appear, we trust, from a very slight considera- 

 tion of the subject, that the magnifying power thus 

 K i*< found, is not the real measure of the assistance afforded 

 by the microscope. Let as take the Case of a compound 

 microscope, with two lenses, and let us suppose that 

 the distance of the object from the object-lens is one 



foot ; the distance of the image behind it 20 feet, Compound 

 the focal length of the eye-glass 1 inch, and the dit- 

 tance at which the eye sees microscopic objects 6 inches. S _^J > _- 

 Hence the magnifying power of this microscope, com- 

 puted on the old principle, is x -y- = 120 times; 



that is, the object apptart 120 times larger than if it had 

 been placed at the distance of 6 inches from the rye. 

 But the object is actually placed at the distance of SI 

 feet from the eye, and the image is not only magnified 

 20 times by the object-gls?, but it is brought to such a 

 distance that the eye can see this magnified image at the 

 distance of 6 inches. Hence the real magnifying effect 



. 20 feet 21 feet 

 by the object-glass alone, is x - = 840 ; 



and when we look at this image with an inch eye- 

 glass, it is again magnified 0' time.*, and the total mag- 

 nifying power is 840 X G = 5040 times, in place of 

 HO, according to the old principle. 



Although this is obviously a correct measure of the 

 benefit which the eye deri\e.-> from the microscope, and 

 of the effect of the instrument, yet it will be said that 

 though the object is placed at the distance of 21 feet 

 from the eye, the eye can advance to it and examine it at 

 the distance of 6 inche*, so that 1 20 times is the measure 

 of the assistance which the yr receives when it has 

 placed itself in the best position t'c r examining the object. 

 This is undeniable, but we might as well say that a 

 telescope dinvUil to a table of logarithms, at the dis- 

 tance of 1000 feet, did not magnify SO times (provided 

 that was its magnifying power) because the observer 

 could advance to the book, and see the figures under 

 a larger anple with his naked 



1 1 we suppose that the microscopic object is placed 

 in a cavity, whose depth u 1 '2 inche?, then if the ol>- 

 doea advance to it, he cannot see it at a less dis- 

 tance than 1 - inches ; so that even on the old principle 

 the magnifying power is 240 times. If the object is 

 placed in a position where the rye cannot advance to 

 it at all ; then, on every principle, the real power is 

 5040. 



VIII. On the MrlhoJ of tim ing and Illuminating 

 Microtcopic Olyectt, 



The art of illuminating microscopic objects is not of O.T the me- 

 less importance than that of preparing them for obser- thod of 

 vation. No general rules can be given for adjusting the v ' w 'ng 

 intensity of the illumination to the nature and charac- f nd ' 

 ter of the object which is to be examined ; and it is m^* 

 only by a little practice that this art can be acquired. me object*. 

 In general, however, it will be found that very trans- 

 parent objects require a less degree of light than thu-- 

 which are less so ; and that objects which reflect white 

 light, or which throw it off from a number of lucid 

 points, require a less degree of illumination than those 

 whose surfaces have a feeble reflective force. 



Most opticians have remarked, that microscopic ob- 

 jects arecommonly seen better in candle light than in day- 

 light.* fact which is particularly apparent, when very high 

 magnifying powers are employed ; and we have often 

 found, that very minute objects, which could scarcely 

 be seen at all in daylight, appeared with tolerable dis- 

 tinctness in candle light. So far as we know, the cause 

 of this has not been investigated ; and as it leads to 

 general views respecting the illumination of microsco- 

 pic objects, we shall consider it with some attention. 



Let LL, Fig. 24, be a single microscope, placed be- Fig. t*. 

 fore the eye at E, and let J be a microscopic object, 



4 



