Monthly Microscopical 
EEE Tale Leo. Olyect-glasses and their Definition. 19 
ably a test for object-glasses of good quality and workmanship, for 
not only may chromatic and spherical errors be detected, but any 
scratches or particles of dust on the surface of the lenses may be 
mapped out. 
It was remarked by Mr. Slack (who may be presumed to be 
familiar with the illumination employed), “that the effect produced 
by Dr. Pigott had no connection whatever with the mode of illumi- 
nation adopted by Dr. Woodward.” As the question at present 
stands, such a defence is easy, for I in common with many others 
may well reiterate the question, What is Dr. Pigott’s method of 
illumination ? The directions given in the paper of November 10th, 
1869, are so vague and scanty, that many having the best glasses 
are unable to develop the beaded structure, and those that do obtain 
it are compelled to find out a method for themselves as Dr. Wood- 
ward has done. 
It is scarcely to be expected that those who have not in some 
degree been practically familiar with the construction of object- 
glasses, can be fully aware of the value of the mercury globule in 
originating combinations. ‘To the optician it is as needful as the 
callipers and straight-edge to the engine-fitter—every glass is sepa- 
rately tested by it. Its familiar readings show whether the work is 
going on right or wrong; by the indications of inward or outward 
coma whether the oblique pencils are correct, and finally, the least 
chromatic or spherical error can be ascertained by its means. It 
may be “ well-known to mathematicians that these globules are not 
perfectly spherical” (and mathematicians will be correct.in all things), 
but setting aside the fact that the more minute the particle the 
nearer it approaches to a true sphere, it happens that shape is not 
of the smallest consequence, or whether it is illuminated by oblique 
light, for it is not the globule but the absolute point of light re- 
flected from it that is used. ‘The diameter of a mercury globule for 
correcting the highest powers from a ¥ upwards is only the one 
jiwe thousandth part of an inch. Perhaps some one who thinks it 
may advance the subject, will be good enough to calculate the size 
of the image of a small lamp-flame set at 4 inches distance, reflected 
from the surface of a convex mirror of ys4o9 of an inch radius. 
Dr. Pigott, by converting the microscope object-glass into a 
species of telescope, and viewing distant and minute discs of light, 
professes by means of the “ Aplanatic Searcher” ! ! ! to have discovered 
spherical error in all our best glasses, to the existence of which 
everyone else has hitherto been blind. Doubtless a very imposing 
or “striking” demonstration may be made out of this, but it is 
easy to demonstrate that by so doing we are setting at naught the 
very qualities and advantages of large angular pencils. The con- 
jugate foci are now so far distant, that large angular aperture no 
longer exists. A difference in the adjusting collar that would pro- 
c 2 
