456 Dr. Draper on a new Imponderable Substance t and a 



effected the dispersion of the incident beam. Now, the lens 

 not being achromatic, the screen /• v had to be placed in an 

 inclined position in order to obtain a neat spectrum-image of 

 the hole in b b, and this was attended with the great advan- 

 tage of elongating the total length of the spectrum, and there- 

 fore increasing the measures. In order to obtain sensitive 

 surfaces of great delicacy the silver plates were first iodized 

 lightly, and then exposed to the vapour of bromine until they 

 attained a full golden yellow. 



In the Plate, the line No. 1, fig. 3, represents the visible co- 

 lorific spectrum; it, with No. 2, serves as an index of com- 

 parison for all the others. No. 2 represents the effect of a 

 spectrum that has not undergone the action of any absorbent 

 medium on the bromoiodized plate, the extreme red tinges 

 the plate white, the extreme violet brown, and all the inter- 

 mediate space is of a rich brownish violet, with a point of 

 maximum action nearly in its centre. The numerical sub- 

 divisions commence with at the extreme red, and are gra- 

 duated on a principle, which I shall explain in a future 

 paper, which makes the spectra of different tithonographists 

 comparable. 



No. 3 shows the spectrum after absorption by the persul- 

 phocyanide of iron, and its corresponding tithonograph. This 

 spectrum is divided into three portions, one of which is red 

 and yellow, a second indigo, and a third violet. But the ti- 

 thonograph exhibits an action far beyond the extreme red, half 

 way through the dark space that intervenes in the middle of 

 the spectrum, both ends of this lower part projecting into 

 dark spaces ; whilst the indigo ray, ordinarily so active, does 

 not tithonize at all. 



Without going into a long descriptive detail of the com- 

 parison of different spectras and their corresponding tithono- 

 graphs, I shall here sum up the results which may be gathered 

 from an inspection of the Plate. 



By the absorbent action of the persulphocyanide of iron, 

 we can prove the existence of invisible tithonic rays beyond 

 the extreme red, — invisible rays corresponding to the green. 

 We can also prove that the indigo-coloured rays of light 

 may exist without tithonic effect. 



By the absorbent action of neutral chloride of gold, we can 

 insulate blue coloured rays of light that are not tithonic. 



The green solution formed by a mixture of bichromate of 

 potash, muriatic acid, and alcohol, enables us to insulate ti- 

 thonic rays of the same refrangibility as the violet, but unac- 

 companied by any light. 



The solution of sulphate of copper and ammonia enables 



