14 
Transactions of the Society. 
portant carve ; it will be seen at once that it is that of a well-corrected 
objective ; it is a semi-apochromatic 1 /4 with the high optical index 
of 18*6 (28*2 being the highest optical index of an apochromatic 1/4 
as yet made) ; it is slightly over-corrected in the red, but is well 
corrected in the blue ; this lens will probably be a good photographer, 
visually it is very sharp (B iii). 
No. 14 is the curve of a very old 1/4 by Andrew Boss (1835), 
consisting of three doublets ; the optical index is only 9 * 5, correction 
(A slight). No. 15 is a 3 /4 by Powell, of 1842; the optical index 
is a trifle higher, and the correction (B iii). No. 16 is a 1/4 by 
Andrew Ross, 1847, which, like the preceding, has three doublets, 
and does not show much colour (B iii slight). No. 17 is a single- 
fronted 1/4, optical index still increasing. £\o. 18 a 1/4 by Powell 
(1875), triple front and back and double middle, optical index en- 
larged (B ii). No. 19 an early cheap Student’s English 1 /4, with 
single front and no correction collar ; it has a lower optical index, and 
a correction (B iii). No. 20, an American 1 /5, about eight years old, 
violently coloured (B ii), but otherwise well corrected, with a higher 
optical index. No. 21, a Tolies 1/4, really a 1/6; this is a curiously 
corrected objective, giving tolerably sharp definition, with a red colour 
(B i), optical index somewhat lower than the preceding. No. 22, a 
Hartnack 1/7 (1867), a blue lens (A). Nos. 23 and 24 are the 
curves of the same objective, viz. a cheap semi-apochromatic 1 /7 
(B iii). No 23 is the curve with a full cone of illumination, and 24 
that with a 3/4 cone. The optical index is low, being the same as 
that of the dry apochromatic 1 /8, but the images it yields are scarcely 
inferior to that lens. 
The curve of Powell’s very fine achromatic 1/4 is not given, 
because it is a straight line, and would therefore be only a repetition 
of No. 10 ; its optical index is 23*2. 
We will next observe the variations in the curves of an achromat, 
semi-apochromat, and apochromat owing to different corrections of the 
eye-piece. An achromatic 4/10 (No. 12, fig. 1) has the flattest 
curve, with the Huyghenian eye-piece. With an under-corrected 
eye-piece (single lens) the curve remains the same in the blue, but it 
is a trifle more bent in the red. With an over-corrected eye-piece 
(compensating) the steepness of the curve is slightly increased, both 
in the red and in blue and violet. 
A semi-apochromatic 1/3 (No. 11) has the flattest curve with the 
over-corrected compensating eye-piece; with a Huyghenian it is a 
trifle more bent, and with an under-corrected single lens it is a good 
deal more bent in the B and G lines. 
An apochromatic 1/2 (No. 10) shows no measurable difference 
between the compensating and the Huyghenian eye-pieces ; but when 
an under-corrected eye-piece is used, there is a slight under-correction 
noticeable throughout the spectrum. 
Most of the above differences are too slight to be represented by 
a drawn curve, unless much amplified. 
