Developed Photographic Image. 163 



ends, it being found necessary in grinding to have an appreciable 

 thickness. I was enabled to calculate the relative absorptive 

 values of each wedge ; and the following Table will give an idea 

 of the degree of accuracy with which they were scaled. The 

 values are given in lengths of a half-inch scale, starting from the 

 calculated zero of the wedge which I have called A. Each of 

 the wedges were reduced to the same scale. The numbers refer to 

 different opacities which were measured. A mean of six read- 

 ings was taken in each case; and in no instance did any reading 

 vary more than *15 from the mean. 





A. 



B. 



€. 



No. I. . 



. 7-15 



7^13 



7-13 



No. II. . 



. 10-21 



10-20 



10-18 



No, III. . 



. 12-44 



12-45 



12-41 



No. IV. . 



. 17-60 



17-50 



17-52 



No. V. . 



. 18-60 



18-52 



18-52 



From careful measurements it was found that the coefficient 

 of absorption for each unit of scale of wedge A for the light with 

 which the measurements were taken was *192. 



The photographs of the rotating star were taken of the full 

 size of the original, only half of the disk being in some cases on 

 one plate. Strips were cut from these negatives, one edge 

 always passing through the centre of the image of the star. The 

 relative transparencies of every J r/ or g" were obtained by com- 

 parison with the wedges. From these values the accompanying 

 curves (fig. 2) have been formed, the ordinate being the transiti- 

 on cy, whilst the abscissa is a measure of the intensity of the 

 original reflected light. Only four results are shown— two ob- 

 tained by wet, and two by dry plates. About thirty were 

 measured with almost identical results, 



Each strip was compared with the wedge by daylight, and 

 also by an artificial monochromatic light. The results obtained 

 by the one were nearly proportional to those obtained by the 

 other ; hence only one curve for each strip is given ; and this 

 was obtained by the latter light. To guard against a false ratio 

 of intensity of light due to the lens, negatives of the star were 

 taken at different parts of the plate, and a mean taken. As the 

 lens used was non-distorting and of long focus, the edge and 

 centre of the plate, when directed towards the sky or on a uni- 

 formly white surface, had sensibly the same illumination. Each 

 portion of the strip cut from the negative whose opacity was 

 to be compared was placed above the wedge, at B, and opposite 

 the slit C. These were clamped together and moved till light 

 from behind, shining through the slit and through the image 

 and the wedge resnectively, appeared of the same brightness on 



M2 



