86 Prof. Magnus on the different Properties of the 



and not only is heat of new colours emitted, but also the inten- 

 sity of some of the rays given out by the bright plate is increased 

 by platinizing, to a greater extent than that of others. 



Which of these effects is the one really produced cannot be 

 decided by experiments upon absorption. Supposing that no 

 rays of different wave-length from those previously existing are 

 produced, we might conclude from the fact that alum trans- 

 mits nearly equal quantities of heat from both sources, that 

 the intensity of those rays which are capable of traversing 

 alum is either not increased at all, or only to a small extent, 

 and therefore that this increase takes place principally in the 

 other colours. 



Prismatic Observations. 



It seemed worth while to try whether any further information 

 could be obtained by means of prismatic analysis. Such an 

 analysis is best made by means of a rock-salt prism and rock- 

 salt lenses. But inasmuch as rock-salt always absorbs some 

 heat, alid as it was possible that it might act upon just those 

 rays with which we were here particularly concerned, the expe- 

 riment was performed without as well as (in order to obtain a 

 better separation of the several rays) with the aid of rock-salt 

 lenses. 



Prismatic Analysis without Lenses. 



In order to make the rays fall upon the prism parallel to each 

 other, several diaphragms were employed one behind another. 



Instead of the circular disks mentioned above, two strips of 

 platinum, 3 millims. wide and 33 millims. long, were used as 

 radiating surfaces. Like the platinum disks, they were fastened 

 vertically by fine platinum wires in the apparatus which served 

 to place them successively exactly in the same part of the flame 

 of a Bunsen's lamp. The first diaphragm was placed, at a distance 

 of 60 millims. from the glowing strip, in the side of a large box, 

 about 1 metre long and 0*5 metre in height, and the same in 

 width. The length of this diaphragm was the same as, and its 

 width a little less than, that of the strip. A second diaphragm 

 of exactly the same size was placed inside the box, at a distance 

 of 100 millims. from the first, and immediately behind it was 

 the rock-salt prism. A linear thermo-pile, provided with a slit 

 only 0*5 millim. wide, served to examine the various parts of the 

 spectrum thus produced. 



In order to prevent the first diaphragm, the one nearest to 

 the glowing strip, or more correctly its edges, getting heated, a 

 double metallic screen was placed between it and the strip. 

 This screen was pushed aside during the observations, and since, 

 in consequence of its nearness to the flame, there was danger of 



