202 Messrs. J. W. Gifford and W. A. Shenstone. [Jan. 12, 



described previously.* These rods were applied when softened by 

 heat to one end of a thicker rod of silica and gradually melted 

 into the larger mass, the larger mass itself being maintained in a 

 semi-molten state during the operation about the part where the 

 addition was made by means of a powerful combination of four 

 " mixed gas " oxy-gas flames. The process occupied many days, and 

 in order to secure uniformity in the finished block, the whole was 

 heated throughout after every interval in the process as thoroughly 

 as possible before further additions were made. During the whole 

 process the greatest care was taken to preserve the silica from dust 

 and even from contact with the workman's hand. The complete 

 absence of particles of foreign matter is of great importance, since they 

 would be apt to cause the silica to devitrify during annealing. 



The large mass of silica thus prepared was found to be not quite 

 homogeneous when the prism cut from it was examined in the spectro- 

 meter. Therefore, it was afterwards securely sealed up in a case 

 made of thick platinum foil to protect it from dust, and heated in an 

 oxy-hydrogen furnace till pieces of platinum wire placed near it in the 

 furnace began to melt. To allow time for the heat to penetrate the 

 silica, which is not a good conductor, the process was prolonged so 

 that the silica was kept at a temperature not far below the maximum 

 for several hours. It was then gradually cooled down by slowly 

 reducing the flame of the furnace during 5 or 6 hours, and finally left 

 to fall to the ordinary temperature after all the openings of the furnace 

 had been closed. 



As it did not seem safe to assume that one prism of silica so 

 prepared would be identical in its optical properties with another, 

 a second prism was constructed in quite a different way. This second 

 prism was a compound prism. It consisted of four flat prisms 

 cemented together one above the other. . This compound prism had 

 angles of approximately 60°, and its faces were 56 mm. high by 

 38 mm. wide (see fig. below). It was made from four slabs of silica 

 prepared by Messrs. Baird and Tatlock. The workman, a trust- 

 worthy man, was told that each slab must be reheated till plastic 

 throughout after it was built up. He worked without any supervision 

 from us, and the results show that a uniform material may be obtained 

 from silica without any difficulty and without any other precautions 

 than those which a careful workman may be trusted to take. The 

 four slabs of silica were rough ground into four prisms, cemented 

 together, and a single prism was cut from the mass and finished by 

 Mr. Hilger. 



When the compound silica prism was tested in the spectrometer, we 

 found that its performance generally was not to be distinguished from 

 that of the simple prism used for the measurements given in the table, 

 * 'Nature,' vol. 62, p. 20, aud vol. 64, p. 45. 



