156 Photographs of the red end of the Spectrum. [April, 



intense action through the violet and blue regions as far as b : from E 

 to there appears to be a minimum of action, and then it rises again 

 with clearly marked bands of increased sensitiveness between C and A, 

 strongest between C and B and a to A. Below A the sensitiveness 

 quickly diminishes. 



I have found the sensitiveness to red differ considerably on plates 

 prepared at different times and have not yet discovered to what this due. 

 With a screen of deep ruby glass in front of the slit, the sensitiveness to 

 red becomes very marked and the maximum sensitiveness is found be- 

 tween D and A, the remainder of the spectrum being either cut off at D, 

 or showing only slight traces of action from E to H, according to the 

 length of exposure. 



On a plate taken, about 8 - 30 A. M. on the 19th March, with the plane 

 Rowland grating in the first order spectrum, using a deep ruby glass 

 screen, exposing 10 minutes, the slit being *02 millim., the sensitive action 

 in the red commences just below D, and seems gradually to increase to A, 

 then diminishing to about X 8,000, beyond which lines are distinctly 

 visible, beyond the Z group, up to about X 8,400. Considering that 

 these photographs have been taken without any special appliances for 

 securing the transmission of the infra red rays, it is probable that with 

 them a much greater extent of the infra red spectrum might be photo- 

 graphed on plates stained with this dye. 



With the concave grating also, using a deep ruby glass screen, I 

 have been able to obtain lines in the infra red region to about X 8,300 

 with much greater dispersion and distinctness than the plane grating 

 will give. The photograph I have with me shows the spectrum from 

 C to A and below. It is taken on a Wratten's ' Ordinary ' plate, a slow 

 kind which unstained would show no action beyond E. It is unfortu- 

 nate that the sensitiveness of the plates falls off so much just below A, 

 so that the infra red spectrum has not sufficient density to print well, 

 although up to A the image shows too much density for the lines to 

 print clearly. The various photographs, however, serve to show the 

 value of the dye as a ready and simple means of photographing the 

 spectrum from C to A. with ordinary dry plates. 



Using plates stained with a special preparation of quinoline blue 

 (cyanin) and sulphate of quinine, Mr. J. 0. Burbank has been able to 

 photograph the infra red rays from A to X 9,900, or to about the limit 

 of Cape Abney's latest map, which was prepared from photographs made 

 on collodion plates containing a greenish form of bromide of silver 

 specially sensitive to the infra red rays. I have tried cyanin prepared 

 in this manner and found that though it is undoubtedly sensitive to the 

 infra red rays, its maximum sensitiveness is between D and B and be- 

 tween B and A its action is much weaker than that of alizarin blue. 



