132 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Since I published this, a still more handy device has been brought out 
by the Geneva Optical Company, of whom Messrs. Botwright and Grey, 
of Clerkenwell, are agents iu this country. It is known as a ‘ Lens- 
measure,’ and is depicted in fig. 20. It is the invention of 
Mr. Brayton, and consists of a simple form of spherometer with a 
multiplying-hand to read off the curvature on a dial. But the latter, 
instead of reading off the mere curvature of the surface, has the readings 
of its dimensions already multiplied by the proper constant for crown- 
glass, namely, 0 ■ 54 ; so that the dioptries of a lens are found simply by 
adding the reading of the two sides. 
I will conclude by expressing my satisfaction that the British 
Association has seen its way to appoint a committee on the subject of the 
measurement of lenses. With Prof. Carey-Foster as chairman, and 
with Captain Abney, of South Kensington, and Mr. Whipple, of Kew, as 
members, such a committee ought to be able to effect some real progress 
in the spread of scientific methods, and so help forward the industry. 
The paper was succeeded by a discussion. 
Mr. G. M. Whipple said he had been much gratified at hearing the 
reference which had been made to the work done at Kew in regard to 
lenses, for it ought to be better known that an attempt had been made in 
this country to create an optical laboratory. He was also very glad to 
know that the Technical Institute at Finsbury was doing similar work, 
and he felt sure that with the appliances they had there and the leisure 
at their disposal — not of Prof. Thompson, but of the senior students 
—many most interesting questions in optics might be dealt with, and he 
hoped solved, which could not be accomplished by those who had other 
duties allotted to them. The measurement of lenses for Microscopes had 
not come within the scope of their operations at Kew ; their work was 
the examination of telescopes for the royal navy, binoculars, gun 
directors, a class of telescope not much known to the general public 
which had been recently brought out, besides smaller glasses used in the 
ordinary work of navigation. Again, the sextants which were employed 
by cadets must bear a certificate, and this had led to increased accuracy 
in make. There were also certain minor instruments examined, of 
which he need only mention one, which required a plane surface. The 
grinding of a plane glass surface was a somewhat difficult operation, and 
it had not been much required until recently, when they had been intro- 
duced and their use rendered compulsory for cadets as artificial horizons 
in working their sextants. This was a matter of economy to save the 
waste of mercury, which was somewhat considerable, when mercurial 
artificial horizons were employed. Plane surfaces of blackened glass 
were therefore introduced instead. These things had been going on for 
some time, but of late the Kew Committee had undertaken the examina- 
tion of photographic lenses, and he must add to the names of the 
gentlemen who had been mentioned as laying down the lines on which 
the work should proceed, that of Major Darwin, formerly director of the 
photographic department of the School of Military Engineering. He 
had now retired from the Boyal Engineers, and had thrown himself 
thoroughly into this question of testing photographic lenses. He seemed 
to have consulted all the authorities he could get at, and had made some 
very ingenious and beautiful pieces of ajqiaratus for the purpose, amongst 
