OF ARTS AND SCIENCES. 473 



Investigations on Ligut and Heat, made and published wholly oe in part with 

 Appropriation from the Rumford Fund. 



XXIII. 



CONTRIBUTIONS FROM THE PHYSICAL DEPARTMENT OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XYIL — PHOTOGRAPHY OF THE INFRA-RED REGION 

 OF THE SOLAR SPECTRUM. 



By William H. Pickering. 



Communicated May 1-1, 1884. 



It has been generally assumed, and indeed distiuctly stated by 

 Abney and some others, that the gelatine dry plate is insensitive to 

 that region of the spectrum lying beyond A. On trial, however, this 

 proved not to be the case, as the following results distinctly show. 

 It was found that there was a great difference in the plates, those 

 made by Allen and Rowell, and those by Walker, Reid, and Inglis, 

 giving the best results, the latter being somewhat better than the 

 former. This result was indicated by experiments on the sensitive- 

 ness of the plates to daylight and gas-light, — the two above- 

 mentioned kinds being the most sensitive of all to the latter, while 

 only moderately so to the former light. The oV)ject of the research 

 was to determine to how great a wave-length the plates were sensi- 

 tive, rather than to obtain a good representation of the lines, A 

 very broad slit was therefore used, and a camera lens of large 

 diameter and short focus. The condensing lens, collimator, and 

 camera lens were each 10 cm. in diameter, and the last of 30 cm. 

 focus. The first two lenses were each of about 90 cm. focal length. 

 The prism measured 10 cm. on a side, and had a refracting angle 

 of 30"^. It was so placed that the rays struck the first surface 

 at a slightly oblique angle, thereby obtaining a dispersion equal to 

 that which would be had ordinarily with a G0° prism, and employ- 

 ing only half the thickness of glass. The camera lens had an 

 angular aperture of 19°, and the slit, as usually used, of 1' 20". 

 It should be stated here, however, that in the earlier experiments, 

 made with a common spectroscope, using the object-glass of the 



