150 Transactions of the Royal Microscopical Society. 
smoke if achromatic, or it appears as through thin horn; but 
coloured hghts play about if the achromatism is at fault. 
It may be interesting to quote the * Philosophical Transactions ' 
here: that if the distance of the image from the original be 100 
inches, the miniature, with an eighth, will be 800 times smaller.* 
Again, the sparkle of light on the miniature of a small thermo- 
meter as well as the visibility of the divisions on the ivory scale 
afford highly instructive lessons on residuary spherical or chro- 
matic aberration. At a distance of 100 inches, if the thermometer 
scale is small, say 30 degrees to the inch, the miniature formed by 
an eighth 800 times as small will be the 24,000th of an inch; 
while the divisions on the miniature exceed the delicacy of Nobert's 
minutest lines. At 100 inches the image or miniature formed is in 
an over-corrected state, and the observing objective should probably 
be placed at " uncovered," or at least considerably over-corrected. 
If the miniature be corrected by closing the collar as much as pos- 
sible, a similar effect should be introduced into the observing objec- 
tive. Indeed, plying both collars at once will soon discover to the 
experimenter the peculiar correlations of the observing and minia- 
ture-making glasses. 
I strongly recommend, however, what I may call the funda- 
mental experiment, — a disk of intense light as small as possible ; 
miniatured from a distance sufficiently great to develope the test 
diffraction rings. 
The appearance of one thick, broad, dull ring surrounding a 
planetary disk, I may say, was the first real insight I obtained into 
the real state of the best glasses I could procure. I then worked 
in a darkened room and with a shaded lamp ; illuminating a minute 
aperture or pairs of apertures in blackened brass plate. These 
apertures were about the yioth of an inch in diameter. But they 
were found afterwards to be a poor and insignificant substitute for 
sun miniatures, at this season so difficult of attainment. 
The experimenter on these methods will occasionally be surprised 
at the very curious results obtained by viewing the same miniature 
vdth equal magnifying powers used differently. 
For instance : 
Experiment. A miniature landscape was formed by a small 
convexo-plane lens ^Vth focal length and a lineal aperture of ^t^s, 
O'OS", placed on the stage, the tube of the microscope being directed 
* The formula calculated by the writer is there given : 
m + 2 -1- - 
from which it follows if m be large, 
whore IV, is the niiiiirrnzing power and d the distance between object and miniature. 
