54 Rev. T. Pelham Dale on the Upper Limit 



v = 1*4988, and the refraction-equivalent 13*1. Dr. Gladstone, 

 calculating from A, gives as a probable value 15*3, which 

 would necessarily be greater than that obtained from the limit. 

 A refraction-equivalent for v obtained from orthobromotoluine 

 showed that for this substance the refraction-equivalent for v 

 was between 14 and 15. If we assume G as the limit, then 

 bromine comes out as 13" 7. I subsequently procured a spe- 

 cimen of bromine. In as thin a film as I could produce, the 

 spectra of bromine and selenium were almost indistinguish- 

 able ; but the tint of bromine is decidedly more orange, and 

 its vapour transmitted rays up to F certainly, and perhaps 

 beyond ; but in the brightest sunlight, and even when mixed 

 with air, there was no indication of violet rays. So far as 

 these very imperfect observations extend, they seem to support 

 the conclusions previously arrived at by calculation. Without 

 venturing, then, to speak at all decisively, the subject seems 

 a promising one and likely to repay further investigation. 



In the absence of data as to the spectrum of liquid chlorine, 

 it is useless to make calculations as to any relations which 

 might be found to subsist between it and bromine. It is, 

 however, of importance to observe that its yellow tint is so 

 similar to that of sulphur, that it is not improbable tbat the 

 same portions of the spectrum would be affected in both. If 

 this on farther examination should turn out to be the fact, it 

 would be one step more towards the attainment of the form 

 of the function /i = <p(A,), which would probably reveal rela- 

 tions of great interest toward determining the chemical con- 

 stitution of bodies. 



In the case of the critical angle the imaginary sine corre- 

 sponds with the change from refraction to total reflexion. 

 And this, again, to alteration of phase in the wave of the 

 polarized ray. Is there anything similar in the limit of refrac- . 

 tion ? I think there is. If a mirror be constructed by melting 

 selenium on a glass slide and then pressing on it a somewhat 

 cooler glass, we shall obtain a reflecting surface on remo- 

 ving the glass, and also another on the corresponding surface 

 attached to the glass. At angles near perpendicular incidence 

 the reflected light has a certain greenish tint. This is more 

 apparent on the side next the glass than on the free reflecting 

 surface, but is very evident in both. It is no doubt to be 

 expected that light not transmitted should in certain cases be 

 reflected. If it be absorbed there must be an expenditure of 

 energy within the medium, which we have every reason to 

 believe acts on the wave-length within the medium and the 

 velocity of its transmission. The importance of selenium is 

 that it is a manageable substance, in which the critical point 



