SELECTION OF A SERIES OF OBJECTIVES. 183 
working distance which you may need. Different investigators 
may choose different series, but no one need have a greater number 
in the series. Economy is to be considered in deciding whether 
we shall choose one or another lens; but this is also consistent 
with the statement that all the elements, including economy, may 
be combined in such a small series. The lowest glass may be any- 
thing from a 1% in. toa 3 in. If of an angle of 20° to 25° it will 
have plenty of working 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 too” upwards, to as wide an angle as is con- 
sistent 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 widest 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 1/1o in. immersion having a constant angle of 
too’. 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 roo” is in reserve. 
Mr. E. M. Nelson’s + view is to give the beginner a 1% in. and 
a 2/3 in.; later on a 1/6 in. may be added, and as a higher power 
a 1/12 in. immersion of 1143 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 
* * How to see with the Microscope,’ 1880, pp. 202, 203, and 206. 
+ Engl. Mech., xxxix. (1884) p. 48. 
