OP ARTS AND SCIENCES. 303 



In the first expei-iments the spectrum was produced by a Rowland 

 flat diffraction grating, mounted on a spectrometer circle. This grat- 

 ing contained 17,000 lines to the inch. The observing telescope of 

 the spectroscope was replaced by a camera and lens. 



Certain photographs were also taken by means of a Rowland con- 

 cave grating of 14,500 lines to the inch, and of 21 ft. 6 in. radius of 

 curvature. With this grating, the amount of light being less and the 

 dispersion greater than in the former cases, the exposure had to be 

 increased. 



In all of the experiments ruby-red glass screens were used in order 

 to cut out all of the more refrangible part of the underlying spec- 

 trum. In some cases a weak solution of iodine in carbon disulphide 

 was used with good effect. 



No difficulty was found in photographing from the A line to wave- 

 length 9900, or to the limit assigned by Abney as the limit of the 

 diffraction spectrum. None of the lines were reversed. A special 

 study of the A group was made, photographs being taken at different 

 seasons in order to see if any changes in the remarkable group of 

 lines constituting the A group could be noticed. No existing map 

 represents this group correctly. Employing the second spectrum 

 produced by a concave grating, 52 lines were observed between 

 wave-lengths 7100 and 8000. In the same space Abney records 

 only 24 lines. Between the head of A and the tail of A, the 

 latter being the single line before the series of doublets begin 

 which is so characteristic of the A group, my photographs show 

 17 lines. These photographs were taken in June between ten and 

 one o'clock. 



These results are of special interest when we consider that Abney 

 has said in a Bakerian lecture, "As a result of these experiments I 

 can confidently state that in no case did the addition of a dye cause 

 any chemical effect to be produced by the rays below A of the solar 

 spectrum, nor has Vogel claimed that they do." 



It is interesting to note that Abney is led to believe that the photo- 

 graphic action, which has been noticed hitherto, by the use of dyes as 

 sensitives, can be attributed to a certain action of nitrate of silver on 

 organic matter. This effect is a bleaching one, and only the more 

 fugitive dyes can produce it. We are led to conclude from Abney's 

 paper, that he believes that only a chemical effect produced in a spe- 

 cially prepared emulsion can be used to reproduce the infra red rays. 

 After many experiments he succeeded in producing such an emulsion. 

 The color of this verged upon the blue. Since the color of plates 



