233 CORRESPONDENCE. [ Sourmal, Oct 1, 1610. 
ciple certainly ought not to be overlooked; though perhaps Mr. Wen- 
ham and others who may be trying to extend our optical resources, | 
may think that in saying the neglect of the principle might (in 
microscopic phrase) occasion “loss of time,” Dr. Pigott is, to say the 
least, putting it very mildly. 
Of Dr. Pigott’s mathematical knowledge we may form an estimate 
from his idea that knowing the ratio which the sine of an angle bears 
to the sine of another given angle, we cannot find the former angle 
without having recourse to logarithms. For some of his other errors 
we ought, perhaps, to hold the printer responsible ; but what will the 
Civil Service examiners say to the following exercise in decimals (and 
there are many of the same kind “carefully calculated ”):—The 
number 1°336 is to be divided by 1°500, and the answer given is 
0:890666; that is to say, Dr. Pigott believes that two expressions 
true to three decimal places only, can be made to yield a quotient 
true to siz places. 
The only tangible result from these papers is the suggestion that 
the greater brilliancy of water lenses is in one case due to the fact 
that more of the pencil of light is lost in the air lens than in the 
other from total reflexion. This many persons may not have observed, 
though self-evident when once attention is called to it. Mr. Wenham 
indeed had already* ascribed the superiority in part to the smaller 
loss of light from reflexions; but I think that he there had only the 
general case in view. Dr. Pigott’s remark applies only when the object 
is artificially mounted (in balsam or some such medium). When the 
object is in its natural state, either lens transmits the full pencil; but 
in this case, too, the immersion lens would lose somewhat less light 
than the other by ordinary reflexions; and from the manner in which 
Mr. Wenham expresses himself it seems likely that it was only this 
general case he had in view in the passage referred to. 
I hope that nothing I have said in this letter may seem unduly 
controversial. It is scarcely possible indeed to criticize such papers 
without some appearance of this kind. Dr. Pigott has come forward 
not only as the censor of all existing object-glasses, but also as the 
possessor of improvements which for some unexplained reason seem 
still to be kept back as secrets. In such a case the only means left 
us of guessing at the value of what is kept back is by examining the 
value of what has been published. 
Your obedient servant, 
S. Lestiz Braxey. 
* In the June No., p. 303. 
