PROCEEDINGS OF SOCIETIES. 
177 
and he would simply point out that Professor Huxley himself had 
since doubted whether the creature was the Bucephalm polymorphus of 
Yon Baer, from certain appearances and observations which had been 
described to him, and also from the circumstance of its being found 
free swimming. The resemblance to an ox he could not at all make 
out. 
Mr. Slack suggested that Yon Baer only used a magnification of 
20 diameters. It was a mistake to suppose the creatures he found 
were tough. 
Mr. Badcock said that his own impression was that the creature 
might belong to the same class of animals as that of Yon Baer, but 
might be a different species. 
Mr. Slack thought it most probable that higher powers might be 
required in order to determine the species. 
The thanks of the meeting were then voted to Mr. Slack and to 
Mr. Badcock for their communications. 
A paper by Dr. G. W. Eoyston-Pigott, "On the Principle of 
testing Object-glasses by the Coloured Images produced by Reflexion 
from a Globule of Mercury ; and on Eidola," was read by the Secretary. 
The subject was illustrated by numerous diagrams, which were ex- 
plained by Dr. Pigott as the reading of the paper proceeded. 
The President said that looking at the subject from an a priori 
point of view, he thought it might be very useful when applied to the 
study of forms of minute bodies which were well known, in order to 
see what kinds of false images were produced, so that it might be by 
analogy inferred what was the real nature of other bodies which were 
only known to us possibly by such kinds of images. 
Mr. Browning said he had followed the paper with considerable 
interest, but he had be^ rather puzzled by the statement that when 
a prism was used good images were obtained, but when a mirror was 
made use of they became confused. Dr. Pigott had, however, ex- 
plained this to him by saying that the mirror was a glass one having 
the back surface silvered. 
Dr. Pigott had often seen with a silvered glass mirror of this kind 
as many as half-a-dozen images of the flame of a candle, so that when 
this was used as a reflector, there were perhaps five or six images 
thrown one over the other in such a way as to obliterate the sharpness 
of outline. 
Mr. Elphinstone inquired if Dr. Pigott had seen in an early 
number of the ' Quarterly Journal of Mathematics ' an article by 
Mr. Munro, " On the Final Interference of Light," which bore upon 
these questions. It was well known that if they put two things 
marked in the same way one behind the other, and then looked 
through them, they would immediately get patterns, as in the case of 
two pieces of wire gauze, and it had often occurred to him that there 
were things seen in this way which, though described as real, were in 
fact only false images of things which showed two sets of lines, one 
behind the other. If they went to a pond and threw in two stones, 
and looked at the rings obliquely where they crossed each other, they 
would see a series of hyperbolas. No doubt the things they should 
