Chromatic Curves of Microscope Objectives. By E. M. Nelson. 13 
excellent apochromatic 12 mm. of Zeiss which has a N.A. of *66 or an 
optical index of 32*0. Keferring to fig. 1, we find that No. 1 has 
an optical index of 17 *7 ; it is an inch by Andrew Boss more than 
fifty years old. (Please note particularly that the curve of this lens as 
well as those of the next four is enlarged three times, and a 3/4 cone 
is employed throughout except in No. 23 where a full cone is used.) 
It shows over-correction in the red, but the focus for D is the same 
as for E, but as we proceed to Gr the under-correction increases 
rapidly. It is a good objective, correction (B ii). No. 2 is a modem 
American inch with the slightly larger optical index of 18*7 ; this has 
been better corrected for rays lower down the spectrum, but it is 
under-corrected in the violet ; this is also a (B ii) glass, and its per- 
formance is brilliant. 
No. 3 is a semi- ape chromatic, its power is somewhat less than an 
inch, but it has a higher optical index of 20 * 0. You will notice that 
it is very well corrected throughout the spectrum, its image is nearly 
colourless (B iii slight), it photographs well, and is an excellent 
object-glass. 
No. 4 is an old inch by J. H. Dallmeyer, consisting of two 
doublets ; it is the most achromatic of any old lens I have met' with 
(B ii slight), and it has the fairly high optical index of 20*6. No. 5 
is an apochromatic as may be seen at once, there is a mere trifle of 
over-correction in the red, it is a splendid objective, with the high 
optical index of nearly 30. (The curves of the next six objectives 
are enlarged twice.) 
No. 6 is an English achromatic 2/3 (1875), it is well corrected 
for the lines B D and E, and is correspondingly under-corrected for 
F and G-. It has a bluish correction (A), and is a sharp lens, but 
will not photograph. 
No. 7 is a semi-apochromatic 2/3, and is a sharp lens, practically 
colourless, which will photograph ; its optical index is higher than 
that of the previous achromatic, being nearly 21. 
Nos. 8 and 9 are the same objective, the aperture of No. 9 being 
reduced by a stop. This lens is a 1/2 of 80° by Wenham, but in 
reality it is a 4/10 with the high optical index of nearly 26, correction 
(B iii). This lens has considerable spherical aberration, but when it 
is stopped down to 60° it performs admirably ; it will not photograph 
with full aperture, as it is slightly under-corrected for the F line. 
No. 10 is a splendid apochromatic 1/2 with the enormous optical 
index of 32 * 0 ; notwithstanding no alteration in focus can be detected 
throughout the spectrum. Mo. 11 is a fine semi-apochromatic 1/3; 
it exhibits a strong correction in the violet (B iii), and it will photo- 
graph. (All the curves are now the actual curves of the objectives.) 
No. 12 is a fine English achromatic 4/10 by Powell (1875) ; it has a 
triple front and back and a double middle ; its optical index is fairly 
high, viz. 20 * 0, and it is a well-corrected lens (B iii). It is typical 
of the best achievement in achromatic days. No. 13 is a very im- 
