Tur NortHERN MicRoscoPist. 
NonaiT: NOVEMBER. 1881. 
A VERIFICATION DEPARTMENT. 
A ieee recent papers by Prof. Abbe upon microscopical matters, 
principally connected with the optical portion of the instrument, 
have led several correspondents to send us their objectives, asking 
for an opinion as to their resolving, defining and penetrating power, 
and as an outcome of this we have been requested by several of 
our friends to open a department for the verification of the various 
measurements by which objectives are generally sold. 
Now, our first thoughts were decidedly unfavourable to the 
views of our correspondents ; but lately we have become converted 
to the belief that by so doing we may be of service to many of our 
readers when engaged in the selection of objectives. 
Professor Abbe has taught us that the resolving power of an ob- 
jective depends entirely upon its numerical aperture (provided the 
corrections for sphericity and chromatism have been well bal- 
anced) and the defining power of a wide aperture is certainly better 
than one of narrow angle. 
In the Journal of the Royal Microscopical Society for October, 
1881, on page 833, line 19, appears the following words :—‘‘ Nar- 
row angled and wide angle objectives, if properly made, all being 
capable of possessing the most perfect defining power.” We do 
not know whether this is to be taken from a theoretical or a prac- 
tical standpoint—certain it is, however, that when speaking some 
time ago to the makers of the most perfect objectives in this 
country as to the supply of a half-inch objective of 40° air angle 
so strongly recommended by Dr. Carpenter for use with the 
binocular, they gave the perfectly gratuitous advice, “You must 
not expect it to define as well as one of the higher angles.” 
Taking this in conjunction with the fact that all low angles we 
have met with would never stand deep eyepieces, and certainly 
have not defined so well as those of wider aperture, we are bound 
to suppose that the sentence we have quoted is from a theoretical 
standpoint, and that a treat, in the direction of a perfect objective 
of low angle, is in store for us yet. 
II 
