^Jo^rnIl.foTrim'^ PROCEEDINGS OF SOCIETIES. 283 
in wliicli bis address had been listened to, declined entering upon the 
Darwinian controversy. 
Mr. Hogg then read a paper, by Brevet Lieut.-Colonel Woodward, 
Assistant Surgeon of the United States army, on " Immersion Objec- 
tives and Nobert's Test-plate," which will appear in the next number 
of the Journal. 
Mr, Hogg said he thought that the paper prints of Nobert's test- 
lines were better than the glass positives received with the paper and 
exhibited to the meeting ; that in his opinion the photographs taken 
with Powell and Lealand's ^^th last year by Dr. Woodward were far 
more perfect, the lines being more sharply defined. The wavy indis- 
tinctness which Dr. Woodward in his former paper said represented 
spurious lines could be seen in all the bands ; on close inspection of 
the two photographic paper prints, which were said to show spurious 
lines in the 12th, 13th, 14th, 17th, and 18th, it was difficult to say 
whether there is any difference between them and that said to resolve 
the 18th and 19th bands into true lines. He had observed a fine series 
of lines barely separated from the rest by a dark broader line in most 
of the bands, which were doubtless due to diffraction ; but another set 
of very fine lines on one side of the 19th band looked more like true 
lines than those believed to be so. These lines also increase the dif- 
ficulty of counting accurately, and consequently he had not been able 
to satisfy himself in a single count. He (Mr. Hogg) would like to ask 
Messrs. Powell and Lealand whether the immersion lens used by Dr. 
Woodward can be used either as a wet or dry objective ; and what 
is the increase of the magnifying power of their y^g^th when converted 
into an immersion lens ? In Mr. Hogg's opinion immersion objectives 
were a great gain to microscopists. 
Mr. Browning said that diffraction spectra would almost certainly 
be produced by Nobert's lines acting as a grating. The lines seen on 
each side of the bands had probably this origin. If spectra over- 
lapped each other, a certain number of the lines would probably be 
lost. He did not think the limits beyond which no lines could be 
resolved could be determined theoretically. It must be decided by 
experiment, and the investigation had to be carried on under great 
difficulties. He suggested thallium as the best source of a pure mono- 
chromatic light. 
Mr. Brooke thought that the bands brought out on the photo- 
graphs referred to by Mr. Hogg, were much more clearly marked 
than on the glass slides, for on both sides of the latter spectral lines 
would be observed. Again, on one side of the plate six lines would 
appear in a band, on the other side fourteen could be seen. These 
appearances were probably due to the oblique direction of the light. 
Mr. Lobb doubted whether the lines on Nobert's test-plate could 
be clearly defined beyond the 16th band by any glass whatever; bnt 
Messrs. Powell and Lealand had constructed a test-object ruled at the 
rate of 100 lines to the xo^oo*^ of an inch. These were clearly defined 
by their ^th and yLth immersion lenses. The podura scales ^vere beau- 
tifully defined by the same objectives, and so were the lines on the aciis. 
Mr. Lealand said that the iVith constructed by his firm could be 
used either as a wet or dry objective. The magnifying power of the 
