898 Profs. Trowbridge and Wood on Infra-Red 



Summary of Results. 



The results obtained thus far appear to be in excellent 

 agreement with theory, and indicate that the method gives 

 reliable experimental data regarding the distribution of energy 

 as a function of the groove form. It indicates that diffrac- 

 tion of the radiation from the reflecting planes prevents us 

 from concentrating all of the energy in a single spectrum, 

 but that with a properly sloped edge we can utilize as much 

 as 70 per cent, of the energy. 



This paper is intended only as a preliminary communication, 

 and the investigation of grooves of other forms will be taken 

 up next. A more exact knowledge of the precise nature of 

 the ruled surface is desired, and preliminary experiments 

 have shown that it can be obtained by making sections with 

 a microtome of celluloid casts of the surface. These com- 

 bined with the microscopical examination with red and green 

 light, and spectrometer determinations of the groove angles, 

 will give us a very complete idea of the furrow. 



In the future it is planned to use more homogeneous 

 radiation, by spectral decomposition of white light with a 

 rock-salt apparatus, and study each grating with a wider 

 range of wave-lengths. This will enable us to pass by 

 gradual stages from the energy distribution obtained with 

 the quartz-rays to that obtained with the C0 2 radiation. 



XCIX. JXote on Infra-Red Investigations with the Echelette 

 Grating. By AUGUSTUS Trowbridge, Professor of Physics, 

 Princeton University, and R. W. Wood, Professor of Ex- 

 perimental Physics, Johns Hopkins University* . 



IN the preceding paper we have reported a preliminary 

 study of the distribution of intensity among the spectra 

 of different orders furnished by the echelette grating. In 

 the present note we propose to show that these gratings give 

 us the highest resolving power that has yet been brought to 

 bear upon the remote infra-red region of the spectrum. The 

 form of the intensity curve of the " Reststrahleii " reflected 

 from quartz, discovered by Rubens and Nichols, is shown 

 in fig. 1, a, which is from a figure given by Coblentz. The 

 longer wave-length maximum is considerably higher than 

 the short wave-length one, and the minimum between is very 

 shallow. A curve of practically the same form has been 

 obtained in all other investigations. In fig. 1, h is shown the 

 curve obtained with the echelette grating. The maxima are 



* Communicated by the Authors. 



