256 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Magnification. — 'Whatever may be the facts in regard to the use of 
high-power eye-pieces to secure the requisite magnification in mere 
tests, for long-continued work over the tube anything in the upper end 
of less than about one inch focal length is unsatisfactory to me. The 
strain upon the eye is certainly less with the medium and low-power 
oculars, and the image is better to my eye, even with the finest ob- 
jectives made. I chose, therefore, such focal length in the objective as 
will give sufficient magnification with a Huyghenian eye piece, amplify- 
ing about ten times as the upper limit. Higher magnification by the 
eye-piece may be useful in testing an objective, and may, it is tme, to 
some persons, be available for long-continued work ; but I am making 
a report of personal experience. The only other thing necessary to say 
here is that usually the less amplification the better, after a suitable 
amount is obtained. Hence neither objective nor eye-piece should be 
of less focal length than will conveniently serve the purpose required. 
For a botanical laboratory a 1/2 in. and a 1/5 in. dry objective are 
the best selection for the common work of students. Occasionally 
higher powers are needed, sometimes running up to the highest and best 
procurable. For these exceptional cases provision should be made by 
having a few objectives at hand, but students need not be furnished 
with them as with those first named. Really serviceable magnification 
seems to reach its limit in about a 1/15 in. or at most a 1/18 in. ob- 
jective. Only in rare cases is anything of higher power than a 1/10 in. 
of best quality effectively superseded — -with me, in nothing but certain 
studies upon bacteria. 
Angle of Aperture. — It appears to me that something similar can be 
said of the angle of aperture. In the matter of difficult resolution with 
oblique light, high class and even medium grade objectives have been, in 
my hands, proportionally successful in just about the order of their 
aperture, though exceptions have been noted. But for most other uses, 
it does not appear that the angle of aperture should be relatively rated 
so high, in the qualities of objectives. It must not be inferred from this 
that wide angles are, in and of themselves, injurious for biological work. 
Other things being equal, I should always prefer them, cheerfully 
putting up with any lack of penetration, and, to a certain extent with 
inconvenient working distance, for the other advantages offered ; but 
crispness of outline, of even the smallest bacteria, depends upon some- 
thing else quite as much as upon the aperture and cost price of an 
objective. These smallest bacteria measure about 1/50,000 in., or about 
the distance apart of the dots from centre to centre of Pleurosigma 
angulatum. We all know that great angle is not necessary upon objects 
of this size. The question is whether excess of angle above a certain 
essential degree is of any importance whatever, or indeed, whether an 
objective of wide aperture is, on this account, especially superior, when 
the illumination is a narrow beam of axial light. When the object is 
too small or too slender to be seen by a narrower angle, no doubt can 
exist even in this case of the essential advantage of the greater aperture, 
but unless one wishes to see the flagellum of a bacillus, or a minute 
structure of a diatom valve, his laboratory work may, perhaps, be just as 
successful with first-class objectives, of less than the widest angle pro- 
curable. Should these of moderate angle possess better definition (not 
