14< J. Waterhotise — ITse of ISosin in Pliotographing tlie Spectrum. [Jan. 



by thorough washing. These films were denser than the first and showed a 

 deep orange colouration by transmitted and a strong pink by reflected light. 

 Examined with the spectroscope no absorption bands were visible, and the 

 spectrum was quite obscured above F. 



As already stated, the absorption spectrum of the dye shows well 

 marked bands in the green, and according to Dr. Vogel's theory, this part of 

 the spectrum should act with increased intensity on the divj bromide plates 

 stained with the dj^e ; though the action on the plate may be expected to be 

 nearer the red than the absorption band of the colour, in accordance with 

 Kundt's law that when non -absorbent media are mixed with an absorbent 

 substance, the absorption baud has no constant position, but is displaced 

 towards the red, in proportion as the dispersion of the added non-absorbent 

 medium increases. 



Dry plates prepared with the coloured bromised collodion and exposed 

 for periods varying from 1 to 5 minutes, to the spectrum given by a minia- 

 ture direct-vision spectroscope of about six inches focus, exhibit after develop- 

 ment a much greater sensibility to the green rays than to the blue, indigo, 

 or violet, the maximum of action being below E, extending to about half 

 way to D, and then decreasing till all action ceases just about D. Above 

 E the action gradually lessens nearly to F, beyond which is a wide band of 

 decreased action extending more than half way to Gr, followed by faint but 

 increased action extending for some distance beyond H into the ultra-violet. 

 The increased action in the yellow and green is strongly marked by its con- 

 trast with the very weak action in the blue, indigo and violet. 



On the dry plates prepared by immersion in a watery solution of the 

 dye, the same general characteristics are observed, but the image is stronger 

 and the band of maximum action somewhat more extended between E and D, 

 towards D, at Avhich point the action ends almost abruptly (Fig. 5). The 

 band of decreased action in the blue just above F and extending about half 

 way to G is very clearly marked. The same decreased action accompany- 

 ing increased sensitiveness for less refrangible rays, has been observed on 

 plates stained with various dyes, but the cause has not yet been explained 

 and further observation is required to elucidate the law regulating its occur- 

 rence. 



It is worthy of note that traces of action in the green and yellow were 

 distinctly visible on the plate before development, though nothing could be 

 seen in the indigo and violet, as is usually the case. This is the only instance 

 in which I have observed this effect, though several colours tried have given 

 increased sensibility for the less refrangible rays. 



A reference to the diagrams in Plate I will show that these results are 

 quite in accordance with Dr. Vogel's theory, and tend strongly to confirm- 

 it. As, however, Dr. Yogel has stated his rule in general terms as applica- 



