302 SIR F. W. DYSON, PROF. A. S. EDDINGTON AND MR. C. DAVIDSON ON A 



Thermometer readings, July 10, 74-4; July 13, 73-7; July 14, 71 9; July 16, 

 72 3; July 17, 72 -3. 



By July 18 we had obtained a sufficient number of reference photographs. Dis- 

 mantling of the instruments was commenced, and the packing was completed on July 21 . 

 We left Sobral on July 22, leaving the packing cases in the hands of Messrs. NICOLAU 

 and CARNEIKO to be forwarded at the earliest opportunity, and arrived at Greenwich 

 on August 25. 



The observers wish to record their obligations to Mr. CHARLES BOOTH and the officers 

 of the " Booth " Line for facilitating their journeys to and from their station at a 

 difficult time. 



PHOTOGRAPHS TAKEN WITH THE 4-iNCH OBJECT GLASS. 



16. These photographs were taken on 10 X 8-inch plates. By suitably mounting 

 the camera it was made possible to obtain seven stars on the photographs, viz., Nos. 2, 

 3, 4, 5, 6, 10 and 11 of the table in 5. Of the eight photographs taken during the 

 eclipse seven gave measurable images of these stars, the other plate (No. 6) taken 

 through cloud only showing a picture of the prominences. 



Plates of the same field taken under nearly similar conditions as regards altitude were 

 taken on July 14, 15, 17 and 18 (civil date). Of these photographs, the second taken on 

 July 14 with two exposures (referred to as 14, a and 14.,,,), two photographs taken on 

 July 15 (referred to as 15, and 15,), two on July 17 (17 t and 17,), and the second 

 photograph on July 18 (18,) were measured for comparison with the eclipse plates. 



17. The micrometer at the Royal Observatory is not suitable for the direct com- 

 parison of plates of this size. It was therefore decided to measure each plate, by 

 placing, film to film upon it, another photograph of the same region reversed by being 

 taken through the glass. A photograph for this purpose was taken on July 18. This 

 plate is regarded merely as an intermediary between the eclipse plates and comparison 

 plates and is referred to as the scale plate, being used simply as a scale providing 

 points of reference. In all cases measurement was made through the glass of the 

 scale plate, adjusted on the eclipse or comparison plate which was being measured, so 

 that the separation of the images on the two plates did not exceed one-third of a 

 millimetre. The plates were held together by clips which ensured contact over the 

 whole surface. This method of measurement was found to be very convenient. Each 

 plate was measured in two positions, being reversed through 180 degrees, and the accord- 

 ance of the result showed that the method of measurement was entirely satisfactory. 



The measures, both direct and reversed, were made by two measurers (Mr. DAVIDSON 

 and Mr. FURNER), and the means taken. There was no sensible difference between 

 the measurers, which is satisfactory, as it affords evidence of the similarity of the images 

 on the eclipse and comparison and scale plates. 



The value of the micrometer screws (both in R.A. and Decl.) is 6" -25. 



18. The results of the measures are as follows : 



