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SUMMARY OF CURRENT RESEARCHES RELATING TO 
(10) Camera Lens (small landscape), maker unknown. — A taken at 
rim of mount at front end. 
A = 247-2 
Fj = 416-2 
F 2 =136-7 
8 X = 534-2 
S 2 = - 1-3 
/ = 138 mm. 
JF = 7 • 248 dioptrics. 
II L = 278-2 
H 2 = 274-7 
k = 3-5 mm. 
No complete examination of any lens for rays of different colours 
lias yet been made, but it lias been found that lens No. 4, examined 
above for red light, gives with green light a different value for k, though 
the lens is sensibly achromatic at the principal focus. 
The most interesting results obtained so far are the facts that in so 
many compound lenses the Gauss points are crossed, the first point being 
beyond the second. And, in the case of one lens (No. 7), a Reichert’s 
objective, the distance between these two points is found to exceed the 
distance between the two principal foci. It seems to be a necessity with 
all wide -angled compound lenses that the aberrations can only be 
reduced to a minimum by widely separating the constituent lenses, with 
the result that the optical centres of the combined lens are considerably 
displaced past one another. 
Since the bulk of the foregoing investigations were carried out, the 
authorities of the Kew Observatory have decided upon undertaking the 
testing of camera lenses, and issuing certificates of merit. In this work 
they have had the benefit of the advice of Captain Abney, F.R.S., than 
whom no one is better able to advise as to what is desired for photo- 
graphic purposes. This is an excellent beginning, but it is curious 
that in neither of tho certificates issued is any information given as to 
the position of the optical centres, or their distance apart. The “ differ- 
ences in focal length ” for red and violet rays are given, but whether this 
means difference in true focal length, or difference in position of focal 
plane, is not stated. What a photographer wants is not agreement in 
focal length, but in focal plane, which is a very different matter, Also 
in the “ A ” certificate issued from Kew, it is proposed to state the 
optical distortion at 25° from the axis, “including astigmat'sm.” But 
whether this means that the test is to include one for cylindricity so 
as to give the direction of the axis of astigmatism in the focal plane, 
or whether this term is being misused to denote spherical aberration, 
does not appear. Certainly no really astigmatic lens could be tolerated 
for an instant in photography, as it would result in all vertical lines 
being out of focus when all horizontal lines were in focus, or some 
similar defect. 
I notice in a recent most admirable article on photographic lenses, in 
‘ Nature,’ by Mr. A. Mallock, the term astigmatism is also used for the 
distance between the primary and secondary foci as produced by oblique 
rays. But this is not astigmatism at all, and has nothing to do with the 
