386 Prof. Draper on the Chemical Action of Light. 



versed by the effect of the ray reflected from the back of the 

 film ; and in those parts wliere the chemical effect is at a maxi- 

 mum, the incident and reflected rays have conspired ? 



If any proof were required that the maxima or minima of 

 chemical effect arise from the superposition of similar or contrary 

 motions, it is foimd in the relative thickness of the bands which 

 have been acted or unacted upon. The second yellow film has 

 a thickness compared with the first as 2:1, the metallic gray 

 film which intervenes having a thickness of 1^. 



But the experiment may be varied by employing a film of 

 iodide of sdver of imiform thickness, and simultaneously permit- 

 ting rays of different refrangibilities to act upon it. This 

 amounts, in point of fact, to the exposure of the sensitive surface 

 to the solar spectrum. Hei'e, however, it is necessary to remark, 

 that the numerous experiments which have been made by the 

 aid of a spectrum formed by a glass prism lead only to deceptive 

 results. Even if due care has been taken to have the colours 

 sufficiently pure, by causing the intromitted light to pass through 

 a fissure not wider than ^gth of an inch before it falls on the 

 prism, we cannot compare the intensity of effect in one of the 

 coloured spaces with that of another. For as we approach the 

 more refrangible region, the colours are unduly spread out ; and 

 with the extension of the space over which they are spread, their 

 chemical power must be coiTespondingly diminished. The diffi- 

 culty can only be avoided by the employment of a grating or 

 lines drawn with the point of a diamond upon glass, which gives 

 a spectrum in which the colom's are placed side by side, in the 

 order of their wave lengths, and without any distortion. 



Attachingdue weight to the remark just made, we may,however, 

 determine many facts in the case by the use of the prism, though 

 there is but little difficulty in obtaining photographic impres- 

 sions of the spectrum formed by a grating, as I have heretofore 

 shown (Phd. Mag., June 1845). When a Daguerreotype plate, 

 iodized to the yellow, receives a prismatic spectrum, no other 

 light simultaneously acting upon it, it is impressed differently in 

 its different parts, and five well-mai-ked regions may be pointed 

 out : — 1st. In the region of maximum heat, beneath the red ray, 

 there is no action whatever. 2nd. In the red ray itself there is 

 action ; for if the plate be subsequently exposed to the vapour of 

 mercury, it whitens at that part. 3rd. In the yellow space the 

 action again ceases. 4th. It I'cappears in the indigo, and indeed 

 is there at a maximum. 5th. It disappears in the extra-specti*al 

 violet. 



The tenns ' inaction,' or ' want of action,' can, however, be 

 scarcely used with propriety, for in reality it is a negative action. 

 It might be said, why should we expect at the ends of the spec- 



