25G 



t lie device ultimately hit upon for producing similar spectra to 

 those blocked out was to pass the light from this central 

 beam through two Thorp gratings of about 14,500 lines per inch. 

 There wen 4 very peculiar properties connected with the passage 

 of light through two gratings of similar pitch. When a parallel 

 pencil of light impinged on the first grating, a pencil of violet 

 rays would be diffracted at a certain angle, and when that 

 pencil met the second grating of the same pitch, since a part 

 of the light would be diffracted at precisely the same angle, it 

 would issue parallel to the first pencil. A red ray would be 

 diffracted by a greater amount ; but since the angle of diffraction 

 at the two gratings was equal, red rays would also issue parallel 

 to the incident pencil ; in fact, rays of all colours would issue 

 parallel to one another, forming spectra just like those which 

 occurred in the microscope between the two parts of the objective 

 if a grating were placed on the object stage. This being so, 

 the same visible effect should be produced if these two Thorp 

 gratings were placed between the two parts of the objective as 

 would be if the spectra from the grating on the stage had not 

 been stopped-out, and, provided certain precautions were taken, 

 it was found that this was the case. 



The precautions consisted in stopping-out the central beam, after 

 the light had passed both gratings, by means of a stop, allowing 

 *>nly one spectrum on each side to take part in the formation of 

 the image. This was necessary in order to insure equality of 

 optical path-length between the rays, which by their interference 

 produced the image. At first it looked as though the law of 

 Abbe previously mentioned had been circumvented; but a simple 

 experiment showed that that was not so, for on revolving the 

 grating on the object stage the grating structure seen in the 

 microscope remained fixed in position. Indeed, it was found 

 that, upon removing the object grating altogether, the grating 

 -tincture seen through the microscope still remained. Clearly, 

 therefore, the object grating had taken no part in the effect. 



