and the Intensities of Lights of different Colors. 





Fio.6. 



gratings with a dividing engine. See fig. 6. The widths of the 

 spaces cut out of these gratings 

 exactly equalled the breadths of the 

 cardboard left in the grating. Grat- 

 ings were thus made having spaces of 

 1, 2, 3, 4 and 5 mm . The cardboard is 

 rendered opaque by coating one side 

 of it with ivory-black in dilute shellac 

 varnish. After it has dried the card- 

 board is well flattened before it is 

 fastened to the surface of a piece of 

 hard wood on the dividing-engine. 

 The cutting edge of the cutter for 

 this work must have a very acute 



angle. I made, one by grinding down a rod of Stubs' steel. 

 Heating this to a dull cherry red and then forcing it into a large 

 ball of beeswax gives the edge of the cutter the required temper, 

 without the necessity of subsequently u letting it down." The 

 blackened side of the gratings 

 was covered with the " Alba 

 tracing paper." The grating 

 was mounted back of an open- 

 ing in a black cardboard screen, 

 so that only the white grating 

 was exposed. Two other black 

 screens, Wh and Y of fig. 7, 

 having openings of the same 

 size as the grating and covered 

 on the back with the translucent 



w 



paper, were 



placed 



on 



O L 



Wh,. 



R<r:7, 



either 

 side of the grating, G. In fig. 

 7, L is the lamp. The window 

 is on the other side of G. The 

 screen S divides the apparatus 

 so that the light of the window 

 W reflected by the mirror M on 

 to the screen Wh cannot fall on 

 G or on Y, which are only 

 illuminated by the lamp L. 



The translucent paper of the grating G and of the screen 

 Y appear orange yellow. The bands of white cardboard of 

 the grating appear cyan-blue. The translucent paper of 

 the screen Wh is white. 



Sit a little to one side of the grating so as not to intercept 

 the light from the window, and look at the grating through 

 the calc-spar prism. Rotate it till the blue bands of the grat- 

 ing are superposed on the orange bands, when, if the surface of 



