32 Scientific Proceedings, Royal Dublin Society. 
The substances examined by this method were bone, muscle, 
and fat, taken from the sheep. The maximum thickness of the 
wedges varied from one to two inches. It was necessary to freeze 
the materials so that the wedges might be accurately cut. No 
attempt was made to secure a uniform current in the 10-inch 
Ruhmkorff coil used, as measurements were only made in the case 
of wedges which had been exposed at the same time and on the 
same plate. The time of exposure varied from one to three 
minutes. Only photographs of the central strip of each wedge 
were used, as shadows of the edges of the upper surface were 
thrown on the plate, owing to the obliquity of some of the rays. 
The time of exposure was varied, as well as the depth of colour in 
the prints, so as to obtain the greatest diversity of conditions. The 
lighter-coloured prints were found to be the most satisfactory for 
purposes of comparison. 
Some of the wedges compared were of different sizes; and in 
such cases, a vertical section of each wedge was drawn to scale on 
paper, and the thicknesses corresponding to different distances 
along the base line in each wedge were easily obtained. 
The first comparison was made between muscle and fat. As the 
result of seventeen measurements made on different prints from 
various negatives, the ratio of the opacities of muscle and fat was 
found to be 2°5: 1. 
The next comparison was made between bone and muscle. In 
the case of bone, variations occurred according as the specimen 
contained different proportions of cancellous tissue and of compact 
tissue. The results given are the mean of all the measurements. 
The ratio of the opacity of bone to that of muscle was found to be 
1:6: 1, as obtained from eleven measurements. ‘This result, when 
combined with the previous one, gives the ratio of the opacity of 
bone to fat as 4: 1. Hence, the numbers obtained, as representing 
the relative opacities of bone, muscle, and fat, are 4, 2°5, and 1. 
