164 Dr. J. W. Draper on the Phosphor ograph 



other. Not unfrequently, in certain experiments the exact 

 conditions of which 1 do not know and cannot always repro- 

 duce, the darkening begins at the upper confines of the 

 green, and slowly passes down to beyond the red extremity ; 

 that is to say, its propagation is in the opposite direction to 

 that which it ought to show on the heat hypothesis. 



Still more, as has been stated, there is a dark space above 

 the violet. Now it is commonly held that in this region there 

 is little or no heat. If so, what is it that has expelled or 

 destroyed the light. 



The experiments above referred to I made with the recently 

 introduced luminous paint. It presented the facts under 

 their simplest form. But I have also tried many other 

 samples, for which I am indebted to the courtesy of Professor 

 Barker of Philadelphia. Among them I may mention as 

 being very well known the specimens made by Dubosc, en- 

 closed in flat glass tubes, contained in a mahogany case, and 

 designed for illustrating the different-coloured phosphorescent 

 lights emitted. They are to be found in most physical 

 cabinets. These, however, do not show the facts in so clear 

 a manner. On receiving the impress of a solar spectrum they 

 present patches of light and shade irregularly distributed. 

 Though in a general way they confirm the statements made 

 above, they do not do it sharply or satisfactorily. 



Dubosc's specimens to which I have had access are enume- 

 rated as follows : — 1. Calcium violet ; 2. Calcium blue ; 

 3. Calcium green ; 4. Strontium green ; 5. Strontium 

 yellow ; 6. Calcium orange. Restricting my observation to 

 the space beyond the red (which, as has been said, presents 

 a bright rectangle in the darkness, about as far below the 

 red as the red is below^the yellow) I found that this rectangle 

 is not given by 1 and 2 ; in 3 it is doubtful ; in 4 it is 

 quite visible ; and in 5 and 6 strikingly so. 



Is the blackening then due to heat ? That it occurs beyond 

 the violet (that is, beyond the lines H) seems to render such 

 an opinion doubtful ; for it is commonly thought that the 

 effect of heat is not recognizable there. And in the phos- 

 phorogenic spectroscope I have used, the optical train (prism, 

 lenses, &c.) is of glass, which must of course exercise a special 

 selective heat-absorption ; but the traces of this in the phos- 

 phorograph I could never detect. 



In the diffraction-spectrum, I had attempted nearly forty 

 years ago to ascertain the distribution of heat (Phil. Mag., 

 March 1857), but could not succeed with the experiment in 

 a completely satisfactory manner, so small is the effect. I 

 exposed a tablet of luminous paint to such a diffraction-spec- 



