4-iO PROCEEDINGS OF THE AMERICAN ACADEMY. 



varied from 12 to 21 degrees, depending on which order was being 

 photographed. The axis of the grating was adjusted to meet the 

 photographic fihri at its middle point. By rotation of the arm, tliis 

 relation remained unchanged, but the focal distance changed with the 

 angle. The camera was accordingly made adjustable along the normal 

 to the grating, and by careful trial, three positions were obtained at 

 which the camera could readil}' be set and clamped, so as to obtain 

 spectra in good focus whose middle points lay near 3900, 5900, or 

 7800 tenth-meters. The whole mounting was enclosed by a wooden 

 structure with doubly curtained door, which the observer could enter or 

 leave during an exposure without fogging the film. 



With this arrangement the spectra were almost entirely free from 

 astigmatism for a space of 25 cm., corresponding to nearly 2700 t.-m. 

 in the first order. P^conomy of light was insured by the short focus of 

 the mounting and the large size both of grating and mirror. 



The spectra were photographed on films 35 cm. long and usually 

 2.5 cm. wide. These were mounted in a holder which bound the film 

 all along both edges and forced it to take such a curvature that the 

 spectra were in focus along its whole length. This holder could be 

 moved vertically by measurable amounts by a couple of large screws, so 

 that several spectra could be photographed above one another on tlie 

 same film. Each film took in over 3800 t.-m. at once, with good 

 definition over almost the whole length, if the adjustments were correctly 

 made. The scale of the photographs was nearly 11 t.-m. to the mil-' 

 liraeter in the first order, and two sharp lines could be seen distinctly 

 separated if they were 0.7 t.-m., or less, apart. 



The films used were from the Seed Company and were coated with 

 their 26x emulsion. A few Eastman Nou-Cui'ling films were also tried 

 and found very satisfactory for wave-lengths less than 5900, For 

 photographing lines in the red, the Seed films were stained with a simple 

 cyanin bath. The writer is glad to have an opportunity of thanking 

 Mr. R. James Wallace of the Yerkes Observatory for the formula, 

 furnished by him, for this staining process, and recommends it to others 

 interested, as a very simple and efficient way of sensitizing plates as far 

 into the red as 8000 t.-m. Mr. Wallace's formula is as follows : — 



Cyanin solution in Alcohol (1 : 500) ... 5 cc. 



Alcohol 30 cc. 



Water 60 cc. 



Ammonia .....,...,,. 10 drops. 



