Testing Object-glasses, By Dr. Boyston-Pigott. 149 
of illumination. Those who may have tried this plan will best 
appreciate the difficulty and trouble attending the experiment. 
The method of miniatures is entirely free from this embarrass- 
ment. 
An object-glass of very fine quality, and in an inverted position, 
is screwed into the sub-stage. 
On black velvet are scattered, from a syringe containing mer- 
cury, a number of mercurial globules ; then, by means of a Eeade's 
prism, a brilliant light is thrown vertically downwards upon them. 
The object-glass to be tested is now employed in the usual way, 
screwed to the nose of the microscope. The two objectives are 
brought to a central position, so that their axes coincide, and the 
instrument is then adjusted to form miniatures of the globules for 
examination. The most beautiful effects are seen under sunlight ; 
and most of those which I shall have to describe are so produced. 
These miniatures will develope appearances of marvellous beauty 
and variety, according to the following circumstances of their em- 
ployment : 
(1) Coincidence of the axes of the observing and miniature- 
making objective. 
(2) Particular corrections, either spherical or chromatic, of either. 
(3) (Which is included in the last), the distances at which the 
original globules are placed from their miniature images. 
It is also worth remarking that the aperture of the miniature- 
making objective should be at least as wide as that of the objective 
to be tested, and of the finest quality obtainable, and furnished with 
a screw-collar. Also the choice of powers is a matter of taste and 
experience. It requires an excellent pair of objectives for diminish- 
ing and enlarging, in order to successfully develope the beautiful 
phenomena caused by the clashing as it were of the waves of light, 
commonly called interferences. As a rule, the observing objective 
should not be more than double the power of the diminishing ob- 
jective used to form the miniature. 
Some of the most beautiful effects have been produced by a most 
excellent 1-J-inch Koss for making the miniatures, and then ob- 
served with a ith Wray and a B eye-piece. Occasionally both 
objectives are used of the same power. 
If it be remembered that at 10 inches an eighth diminishes an 
object 80 times, a power of 800 apphed by the microscope will 
develope too much residuary error : 500 will present a more eligible 
image for examination. 
Various objects may be used for miniatures. A small, bril- 
liantly illuminated opened watch, in motion, is rather a surprising 
test. Fine glasses show the working parts of the watch, balance, 
&c., almost as plainly as the naked eye, under a power of 1000 
diameters. Inferior glasses spread the miniature over with a white 
