45 
angle, it was drawn out into a brilliant line accompanied by 
several other images, which caused much confusion, but it 
was quite visible as faras 170°. All, I think, after it touched 
the image of the cell, were useless.* 
«¢ This objective was tried on that most difficult test, the 
Grammatophora Subtilissima, which it did show, but not 
nearly so well as a sixth of 132°, whose whole aperture was 
found to be quite perfect : the image which it gave was ofa 
shadowy character, contrasting very unfavourably with the 
sharp definition of its companion; a result quite obvious if it 
be considered that the good part of its aperture is only 123°, 
and that the effect of all the rest must be actually injurious. 
This was verified by introducing behind the objective a stop 
* This mode of examining the aperture may, in many instances, be useful 
to opticians, as it can be performed with a camphine lamp. If the ex- 
amining microscope be fixed on the alidad of the instrument, and that to 
be examined on a radial slide, light being transmitted through its eye-piece, 
and both be slided till their focal points be in the axis of rotation (which is 
ascertained by the image of the flame not moving when the alidad is shifted), 
it will be found that the image will continue sharp and distinct if the aper- 
ture be good, till:—1. It begins to decrease in brightness; 2. Its edge dis- 
appears; and 3. Lastly, it vanishes entirely. 
Let & be the are between the points where any of these facts are ob- 
served on each side, and a the aperture of ex. microscope’s objective: 
se (R + a) O 
Wee be tan 3 4 = tan ~—3— a 
x. (R -a) O 
De pan rate hae Se ntaTe gCh  Lhgh art 
we (R-a) O 
Dy ae MEE gr Pm BS eS ei 
The first of these is the least accurate. 
If the objective have the defect just described, the image retains its 
brightness, but becomes deformed after a certain angle. In this way I esti- 
mated by the formula 2, the good part of No. 6 to be = 122°, 75, not far from 
the result given in the text. Four other objectives of large aperture gave 
by it measures according with those of Mr. Lister’s method; though, among 
them also, one of 105 was indistinct for a few degrees. 
