J. H. Gladstone—Refraction Equivalents. 55 
, 
merease of sensitiveness to light for development was ob- 
served. Later I shall hope to give more definite details on 
these points. 
- In the above facts will doubtless be found an explanation of 
red films which 
have caused such wide differences of opinion. And the new 
modes of operating deducible from the reactions here described 
will, I think, be found of extended utility. Silver salts can be 
dyed first and emulsified afterwards, and the ability to color 
the sensitive salt to any shade with certainty and without 
introducing a counteracting influence into the film gives a new 
Power in photochemistry. 
Agt. XITI.—On the present state of our knowledge of Refraction 
Equivalents ; by Dr. J. H. Guapstonsg, F.R.S. 
_ I is well known that every substance has its refractive 
index, but this varies with change of temperature or pressure, 
a by the passage from the fluid to the gaseous or solid con- 
ion, 
_ The specific refraction of a substance, that is the refractive 
Index minus unity divided by the density, does not vary under 
the above circumstances, or varies but little. 
new its specific gravity ; and there the value of the investi- 
8ation would probably cease. : 
