88 PROCESS OF POLISHING AND FIGURING 18-1N. GLASS. 
adually changing by other inclinations and showed by mono- 
chromatic light straight, dark, and coloured bands (fig. 7.) 
Several precautions must, however, be taken in using this test. 
An even temperature is absolutely necessary. The angle of obser- 
vation should not be greater than 70° with the normal, but better 
much less, and the glasses must be as perfectly clean as possible. 
These precautions refer more particularly to compound solar 
licht. When monochromatic light is used, the test may not be so 
delicate, but is certainly of much more practical use, and as will 
be demonstrated correct to less than 1-200,000th part of an inch. 
When one end of the glasses is pressed, or when impurities pre- 
vent them from being exactly the same distance apart, the devia- 
tion from theoretical flatness can be judged and rigidly calculated 
by measuring the curve or bends in the bands in comparison Wi 
the distance between them, which should be as large as possible 
(say) from 4 to } inch. 
For, let A (tig. 8) represent two glasses 5-inch diameter placed 
together and illuminated by a homogeneous yellow light (wave 
length = \ =zxbso) at an angle of incidence ¢ of 30°, then from 
optics it is known that the dark bands appear whenever the thick- 
ness of the film of air 6 is = 5 Asec.@ and the bright ones at — 
— Asec. p, and that consequently the deviation in thickness 
between two dark bands is expressed by « = . sec. f, 80, if thus 
: ‘ x 
a curve in the middle band were by measurement found to be 3 
the deviation from flatness in one or both of the glasses would be 
3 sec. @ = 0.0000C65; and, as even a much smaller displace- 
ment might be observed and measured, defects of less than 
sedews in. can be seen. Having thus obtained my measurins 
scales, my silver film was estimated in a similar manner to the 
above. 
‘An ordinary film of silver once repeated was deposited upo 
one of the glasses and a part of it afterwards removed by strong 
nitric acid. When the bands were next examined at an angle of 
9 
Rogstad Se IeS Lee ee tee oe 
