257 



Whilst, therefore, so far as the theory of the microscopic image 

 was concerned, nothing ont of accordance with the known laws 

 had resulted from this somewhat artificial attempt to circumvent 

 one of them, it was seen that they led to the production of 

 achromatic interference bands. Ordinarily, in interference 

 experiments the bands or fringes were spaced according to their 

 wave-lengths. A number of experimenters had tried to make 

 the bands of different colours overlap or achromatise them. 



The double-grating method was very simple, and apparently 

 new. Mr. Rheinberg concluded by referring to the peculiarity 

 that the result was independent of the distance between the two 

 gratings, since this did not affect the black-and-white effect, 

 although it afforded a convenient means of altering the pitch of 

 the bands. 



Votes of thanks were accorded to Messrs. Wesche and 

 Rheinberg for their communications. 



It was announced that the ordinary meetings were suspended 

 until October 20th, the " gossip nights " being continued in the 

 meantime. 



