and Unmeasured Wave- lengths. 399 



both from its minute amount and feeble intensity (even if 

 we can pass it through a prism, to form a spectrum), it is 

 absolutely inappreciable, in anything like homogeneous por- 

 tions, to the most delicate thermopile, and difficult of attack 

 even by the bolometer. 



We have employed as radiating surfaces, Leslie cubes 

 covered with lampblack and filled with boiling water or 

 aniline, the former giving a radiating surface of a tempera- 

 ture of 100° C, the latter one of 178° C.; and also cubes filled 

 with freezing-mixtures, with the latter of which Mr. F. W. 

 Very, of this Observatory, conducted in the cold days of last 

 March one series of measures in which the radiator was the 

 bolometer itself, at a temperature of — 2° C, and the source 

 radiated to, a vessel filled with a mixture of salt and snow at 

 the temperature of —20° C, thus determining the distribution 

 of energy in the spectrum of a surface below the freezing- 

 point of water. The Leslie cube used in these experiments 

 was filled either with a freezing-mixture or with water, kept 

 gently boiling by a Bunsen burner underneath ; or, again, 

 when measurements from a source at an exactly determinable 

 higher temperature were desired, with aniline, which has a 

 boiling-point of about 178° C. A condensing apparatus 

 connected with the cube in the latter case prevented the 

 escape of the aniline vapour. It was also found possible to 

 keep the cube at any intermediate temperature within suf- 

 ficiently narrow limits by properly adjusting the flame. 



The apparatus is shown in PI. IV. fig. 2. Between the 

 blackened side of the Leslie cube C and the spectrometer- 

 slit S were interposed a large pasteboard screen (a) and a 

 flat copper vessel (b) filled with broken ice, both pierced 

 with apertures slightly larger than the slit, to allow the pas- 

 sage of the rays : and the exposures were made by with- 

 drawing a third hollow screen (c) made of copper and filled 

 with ice, which cut off the radiation of the cube from the slit 

 when it was in place. 



The train for forming the spectrum upon the bolometer- 

 face consisted of two rock-salt lenses L, L, and the rock-salt 

 prism P. Each lens is 75 millim. in diameter, and 350 

 millim. focus for visible rays. For the infra-red rays mea- 

 sured on, the focus is from one to two centimetres greater 

 than this. The prism is made from an unusually perfect 

 piece of rock-salt, and is 64 millim. on a side. With this 

 train, composed entirely of rock salt, and an ordinary eye- 

 piece,' the Fraunhofer lines are very distinctly visible in 

 either sunlight or moonlight. The lenses, prism, slit, and 

 other parts of the train were mounted upon the large spec- 



