Monthly Mi ical 
290 PROCEEDINGS OF SOCIETIES. Journal: Nov. 1, 1810. 
drawn down towards the object-glass, sometimes he had found it 
requisite to separate the objective lenses a little more than usual. 
When the lenses of the searcher were drawn upwards, a finer effect 
was produced, on the contrary, by closing the objective lenses. Some 
little skill was required in getting the best results, because it was 
necessary to use the screw collar very carefully. On looking at the 
podura beading with the glasses in a particular position, an effect would 
be¥produced resembling the figure given by Mr. Wenham. These 
club-headed markings were obtained easily enough by altering the 
corrections with the aplanatic searcher. But with different corrections 
the beaded structure may appear. 
The Chairman inquired whether the screw separated the two lenses 
of the searcher ? 
Dr. Pigott: Yes; but care should be taken in separating these two 
lenses to keep the corrections conjugate, and correlative with those of 
the objective. 
Mr. Brooke asked what the nature of the aplanatic searcher was. 
Was it a combination of two lenses ? 
Dr. Pigott: Both the lenses are formed of crossed lenses, cemented 
to flint-glass concaves. 
Mr. Brooke: What is the focal length of these lenses? 
Dr. Pigott: Combined, about an inch and a quarter; the distance 
-between them about half an inch. 
Mr. Brooke: Are they of equal power ? 
Dr. Pigott: No, the back one is the weaker; and they are com- 
bined in different ways to make them more or less corrected. One 
thing is to be considered, viz. keeping up the flatness of the field. This 
effect will vary much, according to the eye-piece that is in use. 
Professor Huxley, addressing the meeting as a visitor, said, that 
being practically interested in researches such as those which Dr. 
. Pigott was conducting with so much ability, and having seen his 
paper in the ‘ Proceedings’ of the Royal Society, he had been much 
struck by it, and being very much in want of means of looking through 
tolerably thick glass under a high magnifying power, he made it his 
business to ask Professor Stokes about Dr. Pigott. The reply was so 
favourable that he wrote to Dr. Pigott for assistance, by whom he was 
told to apply to Messrs. Powell and Lealand, who made an aplanatic 
searcher for him; and he (Professor Huxley) having used it fre- 
quently, must bear witness that there was no sort of doubt as to the 
wonderful illuminating power it possessed, combined also with the 
great magnifying power which could be got out of a comparatively 
low object-glass by the use of this instrument. But when the attempt 
was made to go farther, that is to say, when the instrument was 
applied to deeper object-glasses (it might be from want of proper 
knowledge in the use of it), it did not seem to be of much use.* It 
was an exceedingly important practical question at the present time, 
* Since the meeting at which these words were spoken, I have had the great 
advantage of applying the “scarcher” to deep objectives under Dr. Royston- 
Pigott’s guidance, and I am disposed to form a very much more favourable opinion 
of its utility—T. H, H. 
