wave-JjEnuths in the invisible pkismatio spectkum. 153 



wliicli they are sujierposed by tlie action of the grating, while tlieir suhseiiiient deviiition is deter- 

 niinable by the bolometer plaiieil at I>, provided they retain sutticieiit energy to ati'ect the instru- 

 luent. It will be seen that, according to this method, all those invisible rays, which are n times 

 the deflnitely known lengtli of some visible ray, are caused to pass togetlier tlirough a slit, and 

 tlien through a prism, which sorts out the ray of the first spectrum from that of the second; that 

 of the second from that of the third; and so on, so that the corresponding index of refraction 

 may be determined by observation; with the eye iu the case of the visible, with the bolometer in 

 that of the invisible ray. 



To illustrate the use of the abov^e described apparatus under somewhat unfavorable circum- 

 stances, let us consider as an example the observations of June 13, 1882, which were taken far 

 down iu the spectrum, where the heat is feeble, and the galvanometer deflection small, requiring 

 a widely open slit. The apparatus iiaving been previously adjusted, and the sunlight ])roperly 

 directed by the siderostat, the visible Fraunhofer line Di, of the third spectrum of the grating, 

 was caused to fall upon the slit S^ of the spectro-bolometer. Then, according to the theory of the 

 grating, there passed, through this slit, rays having the wavelengths — 



0,«.589 (3d sjiectrum — visible.) 

 1//.17S (2d spectrum — invisible.) 

 1//.767 (1st spectrum — invisible.) 



The prism having been removed, and the telescope brought into line, an image of Sa, of the 

 same size as the slit itself, was formed in the focus of the object lens, and on testing with the 

 bolometer, whose face was covered with a cardboard screen jtierced centrally with a 2""" slit, the 

 heat of this image produced a deflection of the galvanometer needle of about 30 divisions. The 

 prism was then replaced on the automatic holder and set to minimum deviation, and the image of 

 the slit, coutaining superposed rays whose combined effect had produced the deflection Just men- 

 tioned, was separated into three similar images* (fig. 2) each composed of nearly homogeneous 

 rays. Of these three bands, only the first or most refrangible, containing the Di line, was visible, 

 and its deviation was found to be 47° 41', agreeing with the value given by the table. It was the 

 object of the experiment to find the place of the lower invisible band, by groping for it ; i. e., to 

 determine its deviation by trials witli the bolometer at intervals surticiently close to avoid the 

 possibility of missing it altogether. According to Briot's formula, the deviation should be 4.5° 

 21', and iu the preliminary search the circle was accordingly set to this reading. Beginning at this 

 point, and exposing the bolometer at every five minutes of deviation, it was found that the max- 

 imum effect was obtained nearer 45° 15'. The apinoximatc position having tlius been found, the 

 slit Si was narrowed to 2""", and the following measurements taken, the horizontal line giving the 

 mean results of a series of thirty expo.^-ures of the bolometer, as it moved through the spectrum 



Table J. — Ddermtnation of the refrnngibility of feiblc heat rays. 



I'ri^iiatic deviation 45^^02' 4.'i° 07' 45" 10' 45° l.V 45° 20' 



Means of galvauonieter readings 4.6 5.6 6.0 5.fi 3.7 



The maximum reading at 4.5° 10' corresponds to a coincidence of the 2 ' bolometer aperture 



with the 2""" invisible image of the slit, whose position is sought. From a subsidiary curve drawn 

 through the points whose co-ordinates are, respectively (x = 45° 02', y = 4.6), (j;=45o 07', y—o.G), 



* Thepe three images, lieiug coinpo.sed of rays of different wave-lengths, could not all he in the same focns of th<' 

 same l«-ns at the same time, since the collimator and objective of this spectrometer were simple lenses. The lenses 

 were adjusted by means of a table ot local distances previously prepared, so as to throw a sharp (invisible) image of 

 the hand to he detected. 



S. Mis. 110 20 



