628 Prof. E. W. Wood on a 



grating-space of 0'5 mm. was determined upon, and a strip of 

 glass was accordingly ruled with this spacing on a dividing- 

 engine. On this scale the echelon was built up, the plates 

 being put in position under the microscope, and cemented at 

 the edges by means of small bits of sealing-wax and a hot wire. 

 Considerable difficulty was found in attaching each plate 

 without disturbing the spacing of the others. The first two 

 or three gratings that were made were not very satisfactory; 

 but some experience having been obtained by practice, an 

 excellent one was finally obtained. Only nine plates were 

 used owing to the opacity of the mica in thicker layers. The 

 battery w T as mounted on a square of cardboard over a rect- 

 angular opening of the same size, a clear space 0'5 mm. wide 

 being left to serve as the first grating-line of zero retardation. 

 The whole number of lines was therefore ten. 



The resolving-power, represented by the product of the 

 number of lines and the order of the spectrum, would accord- 

 ingly be about 500. Obviously the sodium lines/requiring a 

 product of at least 1000 for resolution, were beyond the power 

 of the instrument ; but the two yellow mercury lines, sepa- 

 rated by 2'5 times the distance between the Na lines and 

 requiring a product of only 280, seemed suitable. 



The light from an " end-on " mercury-tube, after passing 

 through a collimating-lens and prism, was focussed on the . 

 collimator of a spectrometer, the green (monochromatic) 

 image of the tube being brought on the slit. On placing the 

 echelon on the table of the instrument the spectra showed 

 clear and sharp, and by turning the grating a little could be 

 brought into either single or double position (see Lord 

 Blythswood and Dr. Marchant's paper, Phil. Mag. Apr. 1900). 

 Faint secondary maxima appeared between the principal 

 maxima, owing to the small number of grating elements. 



By slightly shifting the position of the lens, the yellow 

 light from the tube was now focussed on the slit, when the 

 principal maxima immediately doubled in a most beautiful 

 manner and the faint secondary maxima disappeared owing 

 to overlapping. The distance between the components was 

 about one third of the distance between the spectra. For 

 the sake of comparison, a grating of the same spacing and 

 number of lines was ruled on a piece of smoked glass. (To 

 prevent the iilm from tearing it should be first wetted with 

 alcohol and dried.) 



The slit was illuminated with white light, and a cyanine 

 film placed before it. This cut off all but the extreme red 

 and blue; and it was found that in the first order the grating 

 was unable to separate the extreme red and blue of the 



