255 



so many experts ; but he hoped that his remarks would prove of 

 interest to the novices who were present, and induce them to 

 take up this fascinating study. 



Mr. Julius Rheinberg, F.R.M.S., showed an experiment on the 

 production of achromatic interference bands in a new manner, 

 which formed the subject of a paper he had recently read at the 

 Optical Convention. Certain experiments in connection with 

 the theory of microscopic vision had led to the curious result in 

 question, which amounted in effect to producing in the microscope, 

 on the object stage of which a piece of celluloid or paper with a 

 large perforation had been placed, the appearance as if a grating 

 had been placed over it, the lines appearing perfectly sharp in 

 black and white. It was well known that if a grating were 

 placed on the stage of a microscope, and illuminated by a narrow 

 beam of light, it would diffract the light, so that spectra would 

 be seen above the objective when the eyepiece was removed ; and 

 one of Abbe's laws told us that at least two of these spectra 

 must take part in the formation of the image, if that image 

 was to show any detail, for if one spectrum only were allowed 

 to pass the image would be a mere blur. The object of the 

 present investigation had been to see whether, after blocking 

 out all but one of these spectra which had been diffracted by 

 the object, the other spectra could be replaced or imitated by 

 producing precisely similar spectra in some other manner, and, 

 if so, what would happen. In experimenting with this end in 

 view, an Abbe demonstration microscope was used, in which 

 the objective consisted of two parts — viz. a lens combination, 

 which rendered all rays diverging from any point of the object 

 parallel, and a further lens to bring these parallel rays to a 

 focus. The two parts of the objective were separately mounted, 

 so that any desired apparatus could be inserted between them. 

 By means of a diaphragm just behind the first part of the 

 objective, all the spectra arising from diffraction by the object 

 were blocked out, except the central beam (zero spectrum), and 



