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PROCEEDINGS OF SOCIETIES. 
should work, or endeavour to do so, upon some uniform system of 
measurement, such as that of wave-lengths. With regard to the 
advisability of using a cap over the front of the objective, he had 
found its advantages to be so great that he now considered everything 
he had done in that way before using one to be worth nothing at all. 
In working without a cap, he found that a spurious absorption band 
was obtained, which varied very much according to the strength of the 
solution under observation, and this difficulty was not only entirely got 
over by using the cap, but the actual bands of the spectrum were seen 
much more sharply than before. A cap of this kind would also be 
found of the greatest possible advantage in working without the 
spectroscope, because by cutting off extraneous rays in that way, in 
the examination of crystals for instance, the true colours could be 
ascertained far more perfectly than was otherwise possible. He was 
very pleased indeed to find that others were working in that field, and 
he congratulated Mr. Palmer upon the results he had already obtained. 
He had been working lately, and carrying on some experiments, the 
results of which he hoped to lay before them on some future occasion, 
and amongst other things he had thought that in future the slit should 
be made to act symmetrically, moving from both sides at the same 
time, instead of at one side only. In some cases, also, he found it 
would be better not to make the bright dot move, but to make the 
spectrum itself move, so that the part it was desired to examine might 
always be brought into the centre of the field. These alterations would 
overcome a great many objections and difficulties which had hitherto 
been felt in the way when working with the micro-spectroscope. 
Mr. Palmer said that with regard to the use of the cap, some time 
ago he made up a rather strong solution of permanganate of potash, 
which gave, when properly shown, five distinct bands ; but with this, 
as seen without the cap, the whole spectrum was completely " fogged," 
and the space occupied by the five bands was just one great black 
" splodge," and all the rest was cloudy ; but when the cap was put on, 
it broke up into bands as sharply as possible. 
Dr. Pigott inquired what power objective was used in making these 
observations, and being informed that it was 2 inches, he said the cap 
no doubt reduced the aperture, and he thought the smaller the aperture 
the more it would approximate to a telescope. It was an interesting 
question, whether the contraction of the aperture was the cause of the 
increased distinctness. 
The President said the advantage of the cap was not that it 
reduced the angle of the glass, but that it prevented the reflexion of 
the light from the face of the glass covers back upon the object-glass. 
Dr. Pigott said he had not quite understood where the cap was 
used ; he thought in one case it was placed above the objective. It 
was an interesting point whether it would not be well if placed above 
the lens of the objective. 
Mr. Ingpen said that the effect of the cap was entirely dependent 
upon its position ; if put near the lens it would reduce the aperture, 
but it would not do so at all if it were placed at the focal distance in 
