776 Prof. R. W. Wood on the 



bnrely visible, however. To the right and left are the 

 oblique images, but very slightly broadened by diffraction 

 owing to the width of the reflecting edges, which in this case 

 make equal angles with the surface. 



In fig. 2 a w r e have the appearance of things with a 

 grating of constant *0123 mm. The central image is in- 

 dicated by an arrow, and is bordered on each side by the 

 ordinary grating spectra, which are close together on account 

 of the coarseness of the ruling. They are much fainter than 

 I have indicated on the plate. Well to one side, at an angle 

 of about 40° wirh the normal, we see a ver}^ bright and 

 greatly broadened white image of the flame, accompanied by 

 lateral spectra. These are shown by a coloured plate in the 

 forthcoming edition of my 'Physical Optics.' 



These are not grating spectra, but the first class spectra 

 (as Frannhol'er termed them) due to a single slit, or in this 

 ease to a single; reflecting edge of a groove. With a sodium 

 Hame the appearance is as shown in fig. 2 b. We have in 

 this ease three orders of spectra in the region occupied by 

 the central maximum of the spectra of the first class. Their 

 order is indicated below. One or two orders to the right 

 and left of this group are absent, since they fall in the 

 region of the minimum due to a single slit. They are the 

 '• absent spectra " of grating theory. Other groups of orders 

 appear in the regions occupied by the first class coloured 

 spectra, their intensity being much less, however, than the 

 intensity of the ones falling within the region of the central 

 maximum. The existence of these images of the soda flame 

 shows us that the perfect ion of ruling is such that inter- 

 ference, with a path difference of about 30 wave-lengths, is 

 still taking place. In other words, our grating is acting as 

 a reflecting echelon with steps 15 wave-lengths in height. 

 This was observed only in the case of the gratings ruled on 

 the Rowland machine. Those ruled on the small laboratory 

 machine by hand showed only a confused jumble of over- 

 lapping images, which formed an almost continuous band of 

 yellow light. The grating constant was "0123 mm. in the 

 case represented in figs. 1 and 2. With a larger constant 

 the central maximum of the spectra of the first claes w r as 

 narrower and brighter, the lateral rainbow coloured fringes 

 being less in evidence. 



The wndth of the region in wdiich we have these maxima 

 and minima of the first class becomes less as we increase the 

 width of the reflecting steps of the grating. In the case of a 

 grating of such a small constant as *0123 mm., this region 

 of diffraction of the energy from each individual element 



