770 Prof. R. W. Wood on the 



case so that the exposed bolometer strip may be viewed at 

 any time by means of a low power microscope (shown in 

 fig. 4). 



For work in the ultra-violet spectrum a plate-carrier case 

 replaces the bolometer case BC. The details of this plate 

 carrier are not given, as I have here also followed the 

 construction devised by Schumann. 



The instrument has been in use for several months as a 

 spectrobolometer, and has proved in every way satisfactory. 

 The gain in speed of observation over that possible witli a 

 bolometer subjected to disturbances due to air currents more 

 than compensating for the loss of time in pumping out the 

 instrument. 



No annoyance whatever has been experienced from " leak," 

 though of course the instrument is not perfectly air-tight. 

 The leak, chiefly at the rock-salt window on the slit-case, 

 does not exceed one millimetre of mercury in forty-eight 

 hours. In practice a Gaede pump is run continuously 

 during observation at a speed just sufficient to take up the 

 leak. 



LXXXV. The Eclieleite Grating for the Infra-Red. By 

 R. W. AVooi), Professor of Experimental Physics, Johns 

 Hopkins University *. 



[Plate XVII.] 



ONE of the most important problems in Optics is the 

 question of the distribution of intensity among the 

 spectra of different orders produced by a diffraction grating. 

 Practically no rigorous experimental investigation has been 

 made, owing to the impossibility of determining the actual 

 form of the groove ruled by a diamond point on a glass or 

 metal surface. It is very difficult to learn anything from a 

 microscopical examination, and it is by no means certain that 

 the form of the groove will conform to what we believe to be 

 the shape of the ruling point. It occurred to me that a 

 promising method of attack would be to manufacture gratings 

 with grooves of such large size as to make the determination 

 of their exact form, width, &c. a matter of certainty, and 

 then investigate the energy distribution by means of the 

 long heat-waves discovered by Rubens and his collaborators. 

 By employing the residual rays from quartz and a grating 

 with 3 000 lines to the inch, we should have about the same 

 ratio of wave-length to grating space as obtains in the case 

 of a Rowland grating with 14,000 lines to the inch, and red 



*. Communicated by the Author. 



