On the Behaviour of certain Fluorescent Bodies in Castor-oil. 165 



sities, there can be no further change. Here it can be demon- 

 strated that, between the two points above alluded to, the curve 

 should have the form 



V=pl-<*e- rl , (£) 



where/?, q, r are constants, and I is the original variable inten- 

 sity producing the image. From the last point parallelism 

 would result, and y w r ould become a constant. Theoretically, 

 then, the measure of the varying translucency would be com- 

 pounded of (a), (/3), and a straight line. 



The curves shown above lead us to suspect that this is the 

 practical result of increase of intensity and time. From other 

 experiments, however, I am inclined to think that even where 

 there is no saturation the relation between time and inten- 

 sity is not so simple as has hitherto been imagined. When 

 light actually reduces bromide without the aid of a developer, 

 a compound curve somewhat similar to (a) and (/3) will result. 

 In collodio- chloride printing on glass a like result would oc- 

 cur. Presumably the same also occurs when printing on albu- 

 menized paper. The curves deduced by experiment, and also 

 from calculation, show the reason why in a negative the detail in 

 the shadows and highest lights is more difficult to render 

 faithfully than in the half-tones. They may also show why in 

 a print the details in the first-named portions is liable to be 

 obliterated, even should they be well defined in the negative. 



The curves measured from the dry plates show that bromide 

 of silver is less sensitive to low intensities of light than is the 

 iodide. 



The action of different strengths of developers I propose to 

 treat of in a separate communication, as also the relation between 

 time of exposure and intensity of light. 



XXVII. A Note on the Behaviour of certain Fluorescent Bodies 

 in Castor-oil. By Charles Horner*. 



SOME colouring-matters derived from woods, not showing 

 any fluorescence when dissolved in water, alkaline solu- 

 tions, alum, or alcohol, are found to exhibit this phenomenon on 

 treatment with castor-oil ; whilst other substances, which fluo- 

 resce in alcohol &c, are observed to show this property with 

 augmented intensity. 



To obtain clear solutions, the materials are first boiled in 



alcohol, filtered, evaporated to dryness, and then heated with 



the oil. On transferring some of the prepared solution to a 



test-tube and reheating, the fluorescence disappears as the tern- 



* Communicated bv the Author. 



