330 The Sensitiveness of the Retina to Light and Colour. 



It remains to be seen whether in all cases cooling to very low 

 temperatures destroys the power of these radicles to bestow high 

 dielectric values on compounds containing them when at temperatures 

 near or above their melting points. 



In conclusion, we have again to thank Mr. Petavel for valuable 

 help in taking the above described observations, and reducing the 

 results. 



Note added June 1. 



In their paper "On Capacity and Residual Charge of Dielectrics 

 as affected by Temperature and Time,"* Dr. J. Hopkinson and Mr. 

 Wilson give measurements of the dielectric constant of glycerine for 

 high and low frequencies at ordinary temperatures. The value they 

 obtain is 60 for high frequency and 50 to 60 for low frequency. 

 They also say that glycerine has no residual charge. We obtain an 

 almost identical value for a frequency of 120 at -—50°, but below that 

 temperature the dielectric curve of glycerine, we find, runs almost 

 parallel to that of ice. It is difficult to understand why residual 

 charge (which in ice, the above-named authors say, is considerable) 

 should be the cause of high dielectric value at ordinary temperatures 

 in the case of one substance and not in the case of the other. 



*' The Sensitiveness of the Retina to Light and Colour." By 

 Captain W. de W. Abney, C.B., D.C.L., F.R.S. Received 

 May 10 —Read June 3, 1897. 



(Abstract.) 



The author treats first of the extinction of the sensation of light 

 on the centre of the retina. He made his reduction of the intensity 

 ■of the light falling on the illuminated spot with a new piece of 

 apparatus, which consisted of a gelatine wedge bent so as to make an 

 annulus. He describes this wedge and its graduation, showing how 

 its readings can be utilised, they being proportional to the logarithm 

 of the intensity of light passing through it. 



It is found that the smaller the spot of illuminated surface the 

 less reduction in intensity of the light is required, and that the 

 amount of reduction of the light falling on the spot which just pro- 

 duces no sensation of light is connected with the size of the spot by 

 a simple formula, I = x m , where I is the intensity and x the diameter 

 of the spot. Further, he finds that it is the smallest diameter which 

 governs the necessary reduction in intensity and not the area of the 



* 1 Phil. Trans.,' A, vol. 189, 1897, p. 134. 



