Visibility of the Different Colours of the Spectrum. 717 



piece, as shown in the figure (fig. 1). The edges A were ground 

 extremely sharp, and were slightly less than ^ mm. apart. 



Fi-. l. 



B and C are holes in the tube, and opposite these holes were 

 little plane mirrors with universal motion, which reflected 

 on to the prisms the rays from a white comparison surface 

 situated above and beyond the spectroscope prism. The 

 eyepiece was focussed on the two edges. An eye looking 

 into the eyepiece saw, therefore, a strip of spectrum bounded 

 sharply on each side by white surfaces. The luminosity of 

 the strip of spectrum was measured by closing the slit until 

 it was equally bright with the two white patches on each 

 side of it. The strip of spectrum subtended at the eye an 

 angular height of about 9° and a breadth of about 1°. 



The spectroscope prism was covered with a cardboard case 

 to keep out stray light, and the slit supplied with the 

 instrument was replaced by a better quality micrometer one. 

 Also it was found necessary to insert screens in the telescope 

 and collimator tubes to stop reflexion at grazing incidence 

 from the sides of these tubes; this was at first a source of 

 trouble when work was done at the ends of the spectrum. 



For sources of light two 125 volt, 16 candle-power, carbon 

 glow-lamps were taken. They were run in parallel off 

 storage-cells, and the voltage was always checked before 

 every test. One of these lamps, which we shall call A, was 

 enclosed in a box with a ground-glass window immediately 

 in front of the spectroscope slit ; the other, lamp B, was 

 used for illuminating the white comparison surface, and was 

 placed in a gauze screen 92 or 155 cm. behind the latter. 



The luminosity curves obtained were those of lamp A as 

 modified by the spectroscope employed. Under this modifi- 

 cation is included the different absorption undergone by the 

 different colours of the spectrum in passing through the glass 

 of the instrument, and also the different extension of the 



