72 



Messrs. H. Hartridge and A. V. Hill. 



•do at the same time cause injury to the eye structures that absorb them. 

 The visual rays when present in excessive amount or when coming from a 

 source in a position to form sharply focussed images of filaments, etc., on the 

 retina (eclipse blindness) also do harm, and should therefore be reduced in 

 intensity by suitable neutral grey glasses. 



In the case of daylight the modifying glasses should be worn as spectacles, 

 in the case of artificial illuminants on the other hand they should form globes, 

 so as to limit the rays emitted to those useful for vision. For both purposes 

 the glasses recently perfected by Sir William Crookes are ideal. 



Of the many different glasses prepared by Sir William Crookes, those 

 containing iron in the ferrous state stand prominent for their power of 

 absorbing the infra-red rays. Three glasses of different formulae were sent 

 to us to be tested, and in the case of two of them we were able to contrast 

 the specimen made by Sir William himself on a small scale with samples 

 of a large melt by Messrs. Chance Bros. & Co., Ltd. We also obtained 

 specimens of certain other glasses specially made for spectacles, as we 

 thought a comparison of the properties of these with the Crookes glasses 

 might be interesting. The glasses were examined in the following ways. 



The infra-red radiation was estimated by a simple arrangement of filament, 

 condenser, and thermopile, the latter being enclosed in a box and carefully 

 insulated with wool from radiation from surrounding objects. A comparison 

 of the deflection of the galvanometer with and without the glass gave the 

 summation of the effects of all the radiation emitted by the Nernst lamp. 

 We then limited the radiation to the infra-red region by a gelatine absorption 

 filter which absorbed everything shorter than X 6700. (The construction and 

 properties of this filter will be described later.) Finally we added a trough 

 which contained 7'5 mm. thicknesses of water. 



The transparency to visual rays was measured in a simple comparison photo- 

 meter, the white equivalent being obtained by comparing the absorption of the 

 glass with that of a standardised graduated neutral gelatin wedge. Colour 

 filters were then placed over the eyepiece which limited the spectrum to the 

 red, green or blue as required. 



The ultra-violet transmitted by the glasses was estimated by a photographic 

 method. In front of a fast non-colour-sensitive photographic plate was placed 

 a gelatin filter which removed the visual radiation likely to affect the plate, 

 while it allowed the ultra-violet to pass, the dyes used in preparing the filter 

 being methyl violet and paranitrosodimethyl aniline. Over this were placed 

 the specimens of glass'to be tested, and along the edge of the plate was placed 

 a graduated step-wedge. Light was then allowed to fall perpendicularly on 

 the plate holder and after exposure the plate was developed in the usual 



