ZOOLOGY AND BOTANY, MICBOSCOPY, ETC. 447 



ment that all the elements, including economy, may be combined in 

 such a small series. The lowest glass may be anything from a 1|^ in. 

 to a 3 in. If of an angle of 20° to 25° it will have plenty of work- 

 ing distance and penetration. The next glass should be of 40° angle, 

 or very near it, as this is the maximum normal angle for binocular 

 vision of opaque objects. Its working distance should be enough to 

 allow the use of dissecting-needles under it, and the easy illumination 

 of dry opaque objects. These conditions are found in good glasses 

 ranging from 1 in. to 1/2 in. objectives. The third glass should 

 also be a dry glass, having working distance enough to accommodate 

 work with the animalcule-cages and compressors, and upon rough 

 histological material. Its angle should be from 100° upwards, to as 

 wide an angle as is consistent with the necessary working distance. 

 These conditions are found in glasses ranging from 4/10 in. 

 objectives to 1/6 in. Beyond the three lenses thus generally described, 

 a single immersion lens of w^idest possible angle seems to give all 

 the advantages that can be attained in the present condition of the art 

 of making objectives. 



In the third and fourth of the series, the angle should be the 

 widest consistent with the other conditions specially named, and this 

 is the only demand of the practical microscopist in which, as it seems 

 to me, the phrase ' wide angle ' can have any appropriate place." 



Dr. J. Edwards Smith * says that he has practically, for the past 

 four years, confined himself to the use of four object-glasses, namely, 

 a 1 in. or 2/3 in. of 45° or 50°, a 1/2 in. of 38°, a 1/6 in. immersion, 

 balsam angle ranging from, say 87° to 95°, according to the position 

 of its collar, and a 1/10 in. immersion having a constant angle of 

 100°. Of the last two glasses, the 1/6 in. has a working distance of 

 1/50 of an inch. The 1/10 in. will work readily through covers 

 1/100 of an inch thick. A large amount of his work is on urinary 

 deposits. For the examination of malignant growths and for 

 minute pathology generally, a dry 1/4 in. of 100° is in reserve. 



Mr. E. M. Nelson's f view is to give the beginner a 1^ in. and a 

 2/8 in. ; later on a 1/6 in. may be added, and as a higher power a 

 1/12 in. immersion of 1'43 N.A. "For all working purposes the 

 battery would then be complete, and the microscopist equipped to 

 repeat any results hitherto obtained. As luxuries, a 3 in., 1/3 in., and 

 1/25 in, might be got. It sometimes happened that the high initial 

 magnifying power of the 1/25 in. enabled the observer to find some 

 hitherto unknown object, or portion of an object, more easily than 

 with the 1/12 in. ; but when once found its details of structure would 

 be better made out with the 1/12 in. So far it had not been possible 

 to construct a 1/25 in. as perfectly as a 1/12 in., nor with so high an 

 aperture ; hence it would rarely bear any eye-piece beyond the lowest. 

 The 1/12 in., however, with proper manipulation, would bear the 

 1 in. eye-piece, and then reveal structure that could not be made out 

 with 1/25's, as hitherto constructed. 



* ' How to see with the Microscope,' 1880, pp. 202, 203, and 206. 

 t Eugl. Mech., xxxix. (1884) p. 48. 



