ON DIFFRACTION SPECTRA IN THE MICROSCOPE. If 



the back lens. In place, however, of screwing the objective into 

 the nozzle of the microscope, it may be conveniently screwed on 

 the lower end of an adapting ring carrying an inner tube, moveable 

 as in the small apparatus employed to carry a Nicols' analysing 

 prism, by means of a pin projecting through a slit in the outer ring. 

 When such an adapter is screwed in the nozzle of the microscope 

 tube with the objective in its lower end, the inner tube (rotating 

 through a full third of the circumference of the adapter ring, and 

 commanded by the finger applied to the projecting pin) should 

 reach down with its lower end to the focal plane of the objective, 

 where its rim will afford a holding place for small card diaphragms 

 punched to the exact size of the tube, which may be temporarily 

 fixed with a little wax or other adhesive matter, and changed at 

 will by taking off and replacing with others as wanted. The writer 

 finds this simple plan answer all needful purposes.* 



Object : — 



A plate of glass with groups of lines parallel and crossed, 

 according to the designs of Professor Abbe, is prepared by Zeiss, of 

 Jena. The lines and interspaces are ruled to suit the system aa in so 

 far as dimensions are concerned ; whilst the crossing of the groups 

 at given angles is arranged to produce certain calculated effects. 

 An artificial plate so prepared was used by the writer in his trials, 

 in which the directions of Professor Abbe were strictly followed, 

 with the results given below. 



The centre group on this prepared plate is one of parallel lines — 

 a coarser set ruled with interspaces (from centre to centre of the 

 lines) = '0146 mm. : and between these lines another set dividing 

 the interspaces equally, and therefore '0073 apart. This second 

 set of lines extends only through one half of the length of the first 

 set. Besides this group of parallel lines there are two other groups 



* In the card diaphragms so fixed, holes or slits ot various number and 

 dimensions, according to the intention of the experimenter, must be cut or 

 punched out. 



