322 PROCEEDINGS OF SOCIETIES. ee 
into which the writer of that paper entered; he would therefore make 
a few remarks upon one or two points on which Mr. McIntire had 
touched ; and first as regards the Aplanatic Searcher. He did not 
mean to take any great credit to himself for its discovery ; but if they 
knew the history of its origin, and the great variety of lenses and 
object-glasses inserted intermediately for above six years between the 
eye-piece and the objective, they would still think it capable of im- 
provement. He would mention a similar instance of intermediate 
lenses. Professor Listing professes to get, by means of such lenses 
intermediately applied, a diameter of 17,000, or it may be 50,000, and 
to obtain a definition of Angulatwm in a most marvellous way. The 
Barlow lens is also an instance of an achromatic lens placed inter- 
mediately between the eye-piece and the objective, but forming a 
virtual image. If this lens be applied to the telescope, it amplifies 
the image about one-half without interfering greatly with the defini- 
tion. But if tested on severe objects, as brilliant double points of 
light, those points appear to be brought closer by a spurious disk 
enlargement. 
Dr. Pigott then alluded to the opinions which some years ago 
prevailed as to the structure of the Podura scale, and stated that 
some eighteen years since, he himself had viewed the scale as being 
“hackled” all over with spikes, which Mr. Powell showed distinctly 
to him in 1852 with his new }th. Then with regard to definition ; 
before anyone could speak of perfect definition, he must first of all 
be sure that he could define a point, because every image that is seen 
through the microscope may be considered as an assemblage of such 
points, and therefore the attainment of perfect definition of a point free 
from the false penumbra of the secondary spectrum would be a solu- 
tion of the whole question. The primary question which every micro- 
scopist should ask himself is, Can I define a pomt? If he could do 
that, then he could define an assemblage of points. If he could not, 
a false image was the result. Mr. McIntire had alluded to the mark- 
ings on Lepidocyrtus, which he (Mr. McIntire) considered to be square 
in form. Now twenty years ago, the markings on the Podura scale then 
seen, were thought to be “ghosts,” and in all probability these 
“ ghosts” are but the shadows cast by the real objects. Dr. Pigott then 
described by means of a diagram the false appearances produced by 
the crossing of two strata of spun glass 5,)5,th of an inch in diameter, 
which when placed under the microscope would constantly vary as the 
focus and their inclination, or angles of intersection were changed. 
He also alluded to the conflicting opinions of microscopists as to the 
proper focussing, object-glasses, &c., as regards the particular plane 
preferred. He believed that whatever is seen through a false medium 
prepared by human art, such as a compound microscope, is commingled 
with human error, for it was impossible that any work performed by 
the hand of man should be absolutely free from imperfection like the 
works of Nature; and he admitted this principle into all the investi- 
gations he had made. Instead of depending upon the appearances of 
unknown objects for tests, he simply depended on the appearance of 
known objects ; thus, referring to the effects produced by two super- 
