196 



THE AMEKICAN MONTHLY 



[October, 



of color than as aid to study. Never- 

 theless, there are many cases in 

 which it is quite invaluable, as for 

 instance in studying rocks and crys- 

 tals, and it reveals many details of 

 structure which would otherwise be 

 unnoticed. The prisms should be 

 large, so as to admit sufficient light 

 for brilliant effects, and when the 

 prisms are crossed the field should 

 be quite dark. 



A camera-lucida is quite indis- 

 pensable. A good free-hand drawing 

 from the microscope can be made 

 without the use of a camera-lucida, 

 but even the trained eye is liable to 

 err in delineating objects in this 

 way, and this instrument guards 

 against mistakes from that source. 



A bull's-eye condenser mounted on 

 a stand, an animalcule cage and a 

 compressorium will complete the 

 list of articles that we need mention 

 in this connection. 



It is seldom that the practical mi- 

 croscopist will feel the need of any 

 other accessories, except for pro- 

 ducing brilliant effects of illumina- 

 tion for the mere entertainment of 

 himself or friends. 



The Use of Deep Eye-pieces. — 

 Some time ago a correspondent de- 

 sired an expression of our views con- 

 cerning " the theory now advanced 

 by some microscopists that the true 

 way is to provide one's self with two 

 or three wide-angled and first-class 

 objectives and * eye-piece them up' to 

 the required amplification, instead of 

 obtaining the power from the ob- 

 jective." 



We have never regarded this pro- 

 position as worthy of serious consid- 

 eration, or we should have discussed it 

 in these pages long ago ; but as our 

 correspondent may be one of many 

 who would like to hear the other side 

 of the subject, and as we have seen 

 allusions made to it of late, indicating 

 that the minds of some microscopists 

 are still unsettled concerning it, we 

 offer a few words for their consid- 



eration. If the reader will turn to page 

 89 of the current volume of this jour- 

 nal, he will find the statement that 

 "there is a certain amplification that 

 can easily be obtained by lengthening 

 .the body-tube, or by deeper eye- 

 pieces, which will render visible to a 

 normal eye, every detail which the 

 particular objective can resolve." But 

 the resolving power of an objective 

 is limited by its angular aperture, 

 hence there is a direct relation be- 

 tween the angular aperture of any 

 particular objective and the amplifi- 

 cation necessary to reveal the details 

 which it can portray. In other words, 

 the angular aperture determines the 

 limit of resolving power. The am- 

 plification only separates the images 

 of the particles or lines resolved, so 

 that the rays proceeding from them 

 shall enter the eye at an angle suf- 

 ficiently large for the eye to distin- 

 guish them. 



As an illustration, suppose we have 

 two objectives, a ^-inch of no"; and 

 a ^-inch, also of the same angular 

 aperture. With the same ocular we 

 would have, with the two objectives, 

 an amplification of 200 and 500 dia- 

 meters respectively. Now, an object- 

 ive of 1 10° angular aperture will re- 

 solve about 79,000 lines to the inch. 

 If the ocular employed renders those 

 lines visible with the 34^-inch, giving 

 a power of 200 diameters, the yV'ij^ch 

 giving 500 diameters would not reveal 

 anything more. But, by the use of 

 deeper eye-pieces, we could readily 

 obtain a power of 500 diameters with 

 the ^-inch, and then we would have 

 an exact counterpart of the image 

 given by the ^V'i^^ch — no other detail 

 being revealed than by the lower eye- 

 piece, but all made larger. The ques- 

 tion then naturally arises : if the %- 

 inch, by deeper eye-pieces, can be 

 made to do the work of the 3^-inch, 

 of what use is the ^-inch ? 



In the consideration of this question 

 it will be observed that the angular 

 aperture has no bearing upon it what- 

 ever, — but in passing it may be re- 

 marked that it is only since the time 



