REPORT OF COMMITTEE OX OBSERVATORIES 151 



are found iu the spectrum of the same element the cause of these 

 differences may be investigated. 



I beHeve that the problems which I have named have hardly re- 

 ceived serious consideration hitherto. All are for the purpose of 

 determining accuratel}^ the physical and chemical constitution of 

 at least a few of the heavenly bodies. In the case of the sun itself 

 Rowland's atlas and tables maj- be regarded as only the first though 

 a most important step toward a knowledge of its chemical constitu- 

 tion. A further study of the spectra of the elements would certainly 

 permit 90 per cent of the lines now designated as unknown to be 

 identified. 



\^Fro?n Dr. IV. E. Wilson, Private Observatory , Daramona, Ireland^ 



February 24, 1903, 

 I was very glad to receive your letter of January 28 and to see that 

 there is the prospect of founding a large observatory for investiga- 

 tions on solar radiation and kindred subjects. I am sure such re- 

 search can be profitably carried on only in situations such as you 

 propose. Ireland is certainly not one for such work. I have been 

 experimenting for two years with a new recording bolometer, which 

 I think would give excellent results if it were mounted in a situation 

 where it would get some sunshine. This it does not get here. If 

 you would care to have it tried at your new observ^atory I would be 

 delighted to send it to you. All you would require would be one of 

 Callendar's electric recorders to work with it. It consists of two flat 

 coils of platinum wire blacked and sealed up in an exhausted glass 

 tube. This is enclosed in a brass tube with suitable diaphragm, so 

 that one coil only receives radiation from the sun and a small bit of 

 surrounding sky. The other coil is in the shade. These form the 

 arms of the Wheatstone bridge of the recorder, and it gives a contin- 

 uous record of the radiation. With the form of receiver designed 

 by Callendar the coils were not in vacuum, but merely covered with 

 a glass shade. One coil was black and the other bright, and both lay 

 horizontally, so that the sun was never normal to them, and of course 

 changing from hour to hour. I found that with this old receiver it 

 was quite impossible to calibrate thecurv^e with Angstrom's pyrheli- 

 ometer. Even on a dark, wet day it gave a considerable deflection, 

 by reason of the glare from the clouds. With my new form the 

 curve can be calibrated with the Angstrom instrument perfectly, and 

 by means of a planimeter, which is attached to the pen of the re- 



