188 On Photographs of the Solar Spectrum. [Jan. 20, 



rather an open slit, which injures the definition very much, so that the 

 details of the many lines which I have ascertained to exist in the portion 

 of the spectrum below A are not at all distinct ; but it is possible that 

 means may be found for obtaining better results with moderate exposure 

 and a fine slit ; and I hope to be able to continue the investigation with 

 this object in view. 



I have not been able yet to ascertain in what manner the blue dye 

 acts ; but that it plays an important part in obtaining the results is 

 evident from the fact that on a dry bromide plate unstained the rever- 

 sing effect of the red rays is not observed. I was led to use it from 

 observing, during the course of an investigation of Dr. H. Yogel's impor- 

 tant discovery of the effect of certain dyes on the sensibility of dry 

 bromide-of-silver plates to the less refrangible rays, that on plates stained 

 with aniline red, orauge, green, and blue, the A and other lines in the red 

 were often visible reversed ; and there were traces of reversed action 

 below A, but most markedly on the plates stained with the particular 

 blue dye referred to. As I had not observed it on the plain unstained 

 bromide plates, I attributed the effect to the action of the dyes, over- 

 looking the fact that the plates were all slightly fogged. On repeating 

 the experiments with the blue-stained plates, prepared in a purified bath 

 and very free from fog, the reversed action was not perceptible ; and 

 recollecting the old daguerreotype experiments, the idea suggested itself of 

 trying whether it could be obtained by means of the preliminary expo- 

 sure to light. 



On examining my spectrum negatives, I find that the same reversing 

 action in the red is also to be traced on a few plates prepared with 

 bromoiodized collodion, exposed wet and developed with the ordinary 

 iron developer ; but further experiments are required before I can ascer- 

 tain whether good results can be obtained in this manner. The effect of 

 staining the dry bromide films with other dyes has also to be investi- 

 gated. 



I would further draw your attention to the reversal of the blue and 

 violet rays between H and F. I have observed this effect on all dry 

 bromide plates, whether stained or unstained. It appears to be caused 

 chiefly by .overexposure, but may also be partly due to the action of the 

 alkaline developer. On some plates it is more distinct and of greater 

 extent than on others. I am scarcely sufficiently acquainted with the 

 subject to venture on an explanation of this action of the blue rays, but 

 may remark that they would appear to lose by long and excessive action 

 the decomposing power they first exert on the bromide-of-silver film, and 

 to exercise instead the neutralizing and bleaching action of the red rays. 

 On the single photograph this reversed action of both sets of rays is well 

 illustrated, and the w T hole spectrum is reversed from H to below A, the 

 minimum of action being about F. 



Should you consider the subject of sufficient interest, I trust you will 



