EYE-PRESERVING GLASS FOR SPECTACLES. 
3 
is 35 mm. by 42 mm. The lenses are 52 mm. diameter and 350 mm. focus. The 
condensers are piano-cylindrical, one being double the focus of the other. In order to 
ascertain the exact position of any part of the spectrum I might obtain from the 
radiation from the molten glass, I took photographs on each plate of an ahoy of equal 
molecular weights of zinc, cadmium, tin, and mercury. This alloy gives throughout 
the photographic region lines, the wave-lengths of which are well known. 
The instrument sloped downwards so as to allow the radiation from the surface of 
the melted glass to enter the condensers, prisms, and slit along the axis. To prevent 
the great heat injuring the spectrograph Mr. Nuttall allowed the opening to be 
bricked up, leaving a hole a few inches square in the middle. This was covered with 
an iron plate with a 2-inch hole in it, and over this a quartz plate was fixed. 
Panchromatic films were used. These are sensitive beyond A 7800 in the ultra-red, 
and to the highest ultra-violet rays which will pass through quartz (about A 2100). 
Flexible films had to be used in preference to glass as they had to follow the 
curvature of the focal plane. Many preliminary experiments were made to ascertain 
the extent of spectrum to be recorded, its best position on the films, and the exposures 
needed. The slit of the instrument was generally placed about four feet from the 
molten surface, and it was found that from ten to fifteen minutes were required to 
produce a faint image on development. On each film, immediately before the 
radiation picture was taken, a photograph of the spark spectrum of the quadruple 
alloy was impressed on the film in such a position that the two spectra would overlap 
to a very slight degree. 
No. 1 photograph was taken at the working end of the tank, where the temperature 
was lower than at the other end. An exposure of twenty minutes was given, the 
width of slit being 0'025 mm. 
No. 2 photograph was taken in the same conditions as No. 1, but with an exposure 
of forty-five minutes. 
No. 3 photograph was taken at the melting end, where the heat was fiercest. The 
width of the slit was reduced to 0'1 mm., and an exposure of half-an-hour was given. 
No. 4 photograph, at the melting end, was exposed for one hour. 
No. 5 photograph, at the melting end, was exposed for two hours. 
No. 6 photograph, also at the melting end, was exposed for three hours. 
It was not found practicable to give longer exposures. 
Whilst these experiments were in progress, other experiments at another opening 
at the hottest end were tried to see if X-rays could be detected. Sensitive films were 
wrapped in black paper and then in lead foil in which designs had been cut. ■ These 
were exposed for varying lengths of time, as near as it was safe to put them to the 
radiation from the molten glass, bearing in mind that the heat might affect the films. 
On development, no image of the stencil designs on any of the films could be detected. 
These results confirm those previously obtained by Dr. Burgh —that X-rays are not 
emitted by the highly incandescent molten glass. 
b 2 
