818 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
possible advance narrowest. The ordinary glasses, it is well known, 
only transmit a very small pencil of light of wave-length O’ 3 /i. It 
appears, therefore, that the use of light of wave-length 0*35 /.i is almost 
the extreme point which we can hope to reach without increasing the 
difficulties of the work beyond measure. The use of the wave-length 
0*35 p, instead of the mean wave-length of ordinary daylight, X = O' 55, 
would be equivalent to a raising of the aperture from e. g. 1*40 to 2*20, 
while the use of the wave-length 0*30 p, would raise it to 2*57. Under 
these circumstances, by central illumination, structures would be resolved 
which contained in the length of a millimetre, in the first case 4000 
elements and in the second 4667 (distance apart of elements 0*25 ju and 
0*21 ju respectively), while the corresponding numbers now with aperture 
1*40 and white illumination are 2545 and 0*39 /x. 
Measurement of Lenses.* — Prof. S. P. Thompson, F.K.S., read, at 
the British Association, a paper on “ Some points connected with the 
Measurement of Lenses.” He said that although lenses were used in so 
many departments of practical optical work — as, for example, in the 
making of telescopes, Microscopes, spectacles, and cameras — yet there is 
no uniform system of describing the properties of a lens. Moreover, 
all the text-books of the subject refer only to the particular case of thin 
lenses. He showed how all the properties of a lens could be indicated 
by specifying the position of four points, the two focal points and the 
two so-called “ Gauss points,” where the principal planes of the lens 
intersect the axis of it. No method has previously been given for the 
accurate determination of the Gauss points, and Prof. Thompson 
described an apparatus by means of which he can do this in the case of 
any lens or combination of lenses. The theory of the apparatus 
was also explained in detail. The testing of lenses having become 
a matter of importance in photography, the Kew Observatory has 
recently instituted a special department for the purpose ; but it was 
not proposed to guarantee any great accuracy (say, within a quarter of 
an inch or so) in the measured focal lengths. Prof. Thompson hopes that 
the committee of the British Association, which he has been instrumental 
in establishing, will communicate with the authorities of the Kew 
Observatory, and induce them to carry their measurements to a greater 
degree of accuracy than they have previously contemplated. 
Photographic Optics.| — Mr. A. Caplatzi writes, “ There has just 
appeared under this title a work by Dr. Hugo Schroeder, which will be 
welcomed by practical opticians and amateurs alike. The latter will 
find in it an ample reply to the many requests for information addressed to 
these columns, and the former a practical treatise forming a reliable guide 
in their lucrative business of photo lens-making. In this royal octavo 
of some 200 pages a hard blow has been dealt to rule-of-thumb work. 
Those who will carefully peruse it need no longer work in darkness and 
uncertainty, but can do it in broad daylight and full conviction that 
every step forward will bring them one degree nearer to a successful 
result. And those students who have hitherto derived their optical 
knowledge from the meagre contents of text-books only, will be surprised 
at the number of further considerations requiring attention before a 
* English Mechanic, liv. (1891) p. 36. f Tom. cit., p. 18. 
