660 
SUMMARY OP CURRENT RESEARCHES RELATING TO 
lengths 1:1*8, which I have found a good proportion. But now instead 
of putting the radii of the flint as 1 : 10, or nearly so, I propose to take 
a j>lano-concave and make the radius 24*8. Then I should work the 
hack of the flint, not to an absolute curve, but to an exceedingly long 
concave curve, nearly a plane. This would necessitate the flint lens 
being fitted a short distance from the crown, about two-tenths or so by 
experiment, and I think that this would give a better correction for 
achromatism. I am quite aware that this would give a slight excess of 
spherical aberration to the concave ; but, if necessary, 1 propose to 
correct it after fixing the distance between the lenses that will best suit 
the achromatism. Now, I am rather fortified in my belief that this 
modification will answer, in that my proposed construction will closely 
approximate to the construction of the object-glass of the Lick telescope, 
in which the convex is an equiconvex, and the flint is a double concave 
with a long concave curve at the back, and the two lenses are separated 
and are not in contact. It appears to me that the achromatism would be 
improved by separating the lenses.” 
The Jena Lenses.* — The following is part of a letter from 
“F.R.M.S.”: | — As statements have been made in former numbers of 
this paper impugning the good faith of MM. Zeiss, as to the new 
objective of 1 • 6 N.A., I wrote them requesting an authoritative state- 
ment on the subject. They have very kindly sent me a copy of a letter 
which their Prof. Abbe has written to Mr. Mayall in consequence of my 
communication to them. Their letter is dated 10th September, and in 
it Prof. Abbe says : — “ Please to take notice of a formal assertion from 
my part that the objective has not undergone any alteration whatever, 
while in Jena ; that every lens and every piece of the mounting was in 
exactly the same state at the second departure to London in which it was 
at the first departure.” 
The italics are the Professor’s, not mine. 
Prof. Abbe further states : — “ Though I have not myself looked up 
the lens all the time over, I am in a position to give this assertion 
quite positively on these grounds. 
“ (1) Nobody in the workshop had any sensible interest in making 
an alteration and concealing it to me. For nobody except myself was 
responsible for whatever defect of the objective. The computations had 
been made under my personal direction, and I had approved of the 
optician’s work after execution. If a defect of any kind had happened 
to come out afterwards, the fault would have been mine only. 
“ (2) Nobody could try to change or improve the system without 
consulting me, because no other person was au fait with regard to that 
particular construction.” 
Prof. Abbe further states: — “The objective had not been tried 
photographically by us, neither Van Heurck’s sample, nor the other one, 
I was therefore quite prepared to admit that a 4 chemical ’ focus could 
exist, owing to an insufficient approximation in uniting the violet ray 
with the other rays (in our computation) under the condition always 
that in Van Heurck’s sample the same defect must exist , as both objec- 
* Engl. Mech., Oct. 1890, p. 124. 
f Published, however, without the authorization of Prof. Abbe. — E d. J.R.M.S. 
