REPORT OF THE SECRETARY. 63 



While this number seems small in comparison with the whole number of holo- 

 graphs taken, it nevertheless represents even more work. In this particular work 

 the history of the year is one of continual change and improvement and many of 

 the holographs of the earlier part of the year have been reduced by three different 

 processes, each of which involves three distinct photographic steps, and in conse- 

 quence the 39 final line spectrum photographs stand for more than 200 finished 

 photographs, and as many more which are experimental. 



After a large amount of experimental work, a process has finally been perfected 

 which is fairly satisfactory, and has been adopted provisionally as a working 

 .method. Experiments, however, are still in progress with a view to further modifi- 

 cation and improvement. 



In addition to this bolometric work proper, experiments on three special methods 

 of investigation of the infra-red spectrum, have formed a considerable portion of 

 the year's work : 



(1) Preliminary experiments on the measurement of wave-lengths in the invisible 

 spectrum by interference methods. 



(2) Experiments on photographing the invisible spectrum by the aid of phos- 

 phorescent films. 



(3) Preliminary experiments on bolometric investigation of the infra-red normal 

 (grating) spectrum. 



I. During the month of July, 1N!)2, a series of preliminary experiments was made 

 on a method of measurement of wave-lengths in the invisible spectrum, with special 

 reference to the establishment of a few datum points with great accuracy. The 

 apparatus employed for this purpose, which was kindly lent by Clark University, 

 was a modification of the inferential wave comparer used with so much success by 

 Prof. Michelson. in the establishment of a wave-length standard. 



As the method of working was entirely new, considerable time was required to 

 put the apparatus in working order. Some preliminary results were obtained in 

 the region just above " A," by Mr. Wadsworth, in whose hands I placed it, but the 

 work was interrupted by his departure for Paris, aud has not since been resumed. 

 The accuracy and practicability of this method of determining wave-lengths has, 

 however, been demonstrated, and it is hoped time will be found in the near future 

 to continue this work. 



II. During the mouth of October I made a number of experiments to determine 

 the practicability of photographing the infra-red spectrum directly with the aid of 

 phosporescent films. 



After considerable experimentation on the best method of working, a number of 

 photographs of the invisible portion of the spectrum extending as far as wave-length 

 1-5 fj. were thus obtained. Although the detail is very much less than that obtained 

 by the bolometer, the method is valuable in furnishing a general check on the 

 results of the more analytical method. With greater care in the preparation of 

 films, still better resuls could be obtained. Other films sensitive to heat rays were 

 tried, particularly those containing a salt of mercury, but without adequate results. 



III. During the winter months of January and Febiuary, in which regular bolo- 

 metric work was almost entirely interrupted by bad weather, attention was devoted 

 to the theoretical investigation of a method of bolometrically investigating the nor- 

 mal grating spectrum, the essential feature of which was the employment of a 

 "lifting" prism, by the use of which the superposed spectra were to be avoided. 



After determination of the best instrumental conditions, provisional apparatus 

 was constructed and installed, and a few experimental holographs taken. The 

 approach of good observing weather then necessitated its removal. A paper 

 describing the method aud containing the essential results of the investigation of 

 the instrumental conditions has been prepared for publication. 



C. What might almost be said to have been the chief work of the Observatory for 

 the year has been the improvement of the apparatus and instrumental conditions 

 of working. 



