EYE-PRESERVING GLASS FOR SPECTACLES. 
11 
aluminium screen, R, with a hole in it one centimetre in diameter. The vacuum must 
be a rather high one, about 40 millionths of an atmosphere.* 
The whole apparatus is closely packed with cotton-wool, so that no radiation can 
get to the black disc but that which comes through the window opposite. The box 
containing the radiometer balance is firmly attached to the main wall of the house, to 
avoid as much as possible interference from vibration caused by movements in the room. 
A spot of light reflected by the mirror, S, from another luminous source is received 
upon a graduated screen one metre distant in the usual manner. 
Testing Synthetic Glasses for Diathermancy. 
The mode of procedure is thus :—The mica is put in its place and the lamps started. 
In about ten minutes the zero is adjusted by means of the rotating stopper. When 
the spot of light is at zero the shutter, 0, is raised, and the extent of the deflection 
noted. At the end of the first half swing the shutter is lowered, and the whole is 
left, at rest until the light is again at zero. The glass under test, T, is now put in its 
carrier and slid into place, and the extent of deflection of the spot of light noted when 
the shutter is raised. The amount of deflection with the same piece of mica inter¬ 
posed is thus obtained with many different kinds of glass, and from the data the 
order of obstruction to heat rays can be calculated for each. It is a necessary 
precaution to verify the readings once or twice, and to allow the spot of light to come 
accurately to zero. It is inclined to shift if observations are repeated too rapidly, 
owdng to the retention of heat by the blackened face of the radiometer disc, and the 
consequent repulsion between it and the front of the bulb B. This effect soon goes 
off if a little time elapses between the different observations. 
The deflection of the spot of light when the dark mica alone is interposed gives the 
effect of the total heat ray, and the lessened deflection when the glass under test is 
also interposed is a measure of the heat it cuts off. By dividing the scale divisions 
traversed by the luminous index when both glass and mica are in the path of the heat 
ray by the number of scale divisions traversed when the mica alone is interposed, the 
result gives the amount of heat obstructed. 
Addition of Absorbing Media to the Soda Flux. 
The first point to be settled is the effect of dissolving various metallic oxides by 
fusion in the clear colourless glass. The metal is added in the form of oxide, nitrate, 
or other salt, according to which is easiest to obtain pure. Unless oxidation of other 
ingredients is to be avoided the nitrate is preferred, as the copious liberation of gas 
during the fusion stirs up the fused mixture and assists in making it homogeneous in 
a much shorter time. 
* ‘Phil. Trans. Roy. Soc.,’ 1S76, Part I., p. 301 (the Bakerian Lecture), and ‘Roy. Soc. Proc.,’ vol. xxv., 
