PROCEEDINGS OF SOCIETIES. 
37 
The Mode of Recording Absorption Spectra. 
To the Editor of the 1 Monthly Microscopical Journal.’ 
New York, May 26, 1875. 
Sir, — I write in advocacy of Mr. Sorby’s proposal that the absorp- 
tion spectra should be recorded in wave-lengths, having employed 
this plan for nearly two years. I have, however, abandoned the prism, 
and observe all spectra with the diffraction grating, and obtain the 
wave-length directly by the well-known formula A = 8 x sin. 6. I 
have tried a number of gratings both transmission and reflecting, and 
find that a transmission grating containing between four and five 
thousand lines to the inch, is best adapted to absorption work. The 
one I now use contains 4320 lines and was ruled upon Mr. L. M. 
Rutherford’s machine. With this grating the value of 0 is 5° 45" for 
the D line. 
I enclose a diagram which I use in recording spectra. Will you 
have the kindness to forward it to Mr. Sorby ? and oblige 
Your obedient servant, 
Henry G. Piffard, M.D. 
[On this letter being forwarded to Mr. Sorby, the following reply 
was transmitted.] 
2, Cambridge Villas, Buxton, June 10, 1S75. 
Dear Sir, — I am much obliged by your having sent to me Dr. 
Piffard’s letter. I have long known the special advantages of a 
diffraction grating, but of course prisms have other very important 
advantages, and are in many respects made more convenient to use 
with a microscope. What led me to propose the method described in 
my late paper was the belief that by observing with a prism and then 
so easily reducing the observations to wave-lengths, we should to a 
great extent unite the merits of both systems, and avoid their dis- 
advantages. 
Yours very truly, 
H. C. Sorby. 
PROCEEDINGS OF SOCIETIES. 
Royal Microscopical Society. 
King’s College, June 2 , 1875. 
Charles Brooke, Esq., F.R.S., Vice-President, in the chair. 
The minutes of the preceding meeting were read and confirmed. 
A list of donations to the Society was read, and the thanks of the 
Society were voted to the donors. 
Mr. J. W. Stephenson said he had placed upon the table a number 
of copies of a scale to be used for the measurement of the angular aperture 
of objectives. He had caused them to be printed, thinking that they 
