38 Profs. A. Gray and J. J. Dobbie. Connection between 



second change of hue. Again, after looking at bine for thirty seconds 

 he seeks the third change of hue. The next step is to trace the 

 violet to its limits. After this he works through the spectrum back 

 again, fatiguing the eye with violet before finding the blue, and so 

 on, ending with the determination of the limits of the red. The 

 degree of fatigue is so slight that he is quite unconscious of it. 



The seventy cases examined in this way agree as to the number 

 and mean position of the changes of hue, but they may be divided 

 broadly into those whose colour sensations overlap and those whose 

 colour sensations do not overlap, i.e., those who find the changes of 

 tint occur in the same place when working from red to violet as when 

 returning from violet to red. 



The first class includes persons both educated and uneducated 

 whose avocations require them to compare colours. The second com- 

 prises all who fail with the closer shades of Holmgren's wools. 

 Details are given of some in whom the green and violet are so far 

 extended into each other that they see practically no pure blue, and 

 it is suggested that these, and other differences in the relative 

 intensity and extent of the colour sensations may account for the 

 divergence of opinions among writers on the subject. The paper 

 concludes with an account of five cases of red-blindness. 



"On the Connection between the Electrical Properties and 

 the Chemical Composition of different kinds of Glass." 

 By Professor ANDREW GRAY, LL.D., F.R.S., and Professor 

 J. J. DOBBIE, M.A., D.Sc. Received February 7, Read 

 February 17, 1898. 



The experiments and results described in the following paper are 

 a first instalment of work we have undertaken with a view to finally 

 determining, if possible, the circumstances which affect the con- 

 ductivity and specific inductive capacity of glass. It appeared from 

 some experiments which were carried out by Professor T. Gray and 

 ourselves some years ago,* that it might be of interest to have a 

 number of glasses specially made up with a view to testing some of 

 the conclusions then arrived at. 



A result previously obtained by Professor T. Gray had shown that 

 potash and soda lime glasses have a higher conductivity than flint 

 glasses; this result had also been arrived at by Dr. Hopkinson. In 

 particular it seemed desirable to ascertain whether by increasing the 

 amount of lead oxide and diminishing the amount of soda, the con- 

 ductivity would go on diminishing. We have experienced great 



* ' Roy. Soc. Proc.,' No. 231, 1884. 





