3(34 



Transactions of the Society. 



Fig. 69. 



Zeiss of Jena, for the purpose of exhibiting these phenomena. The 

 apparatus comprises four distinct appliances : — 



(1) An opaque disc to be placed in the distant focus of the sub- 

 stage condenser to define the source of light. It takes two forms. In 

 the one form it is provided with a narrow opening " 4 in. long and 



0'018 in. wide, as shown in tig 6U. In 

 the other form it is provided with a cir- 

 cular opening of " 022 in. diameter. With 

 this second form we shall not this evening 

 be concerned. 



(2) A test plate or diffraction grating, 

 which is sold under the name of Abbe's 

 Diffractions Platte, and serves as the ob- 

 ject viewed in the various experiments. 

 This slide is provided with three groups of 

 rulings. The central one, which is the one 

 principally used in the Abbe experiments, 

 is ruled in one half with a coarse grating 

 having 1750 lines to the inch, and 31 lines in all. In the other half 

 it is ruled as a fine grating with 3500 lines to the inch, and 61 lines 

 in all. The two flanking groups are ruled with about 2400 lines to 

 the inch, and each of them comprises two gratings laid across one 

 another. In one group the angle of intersection is a right angle, 

 in the other an angle of about 62°. The two component gratings 

 are ruled in separate films of silver, and are separated from one 



Fig. 70. 



another by about 0'0004 in. We shall not, however, have much 

 occasion to refer to them in the course of this discussion. By way 

 of completing the description of the principal group, it may be added 

 that the ruled line has a uniform breadth of about one fourteen- 

 thousandth part of an inch. Sufficiently for present purposes the 

 design and proportions of this grating are shown by fig. 70. 



(3) The next appliance consists of a series of diaphragms with a 

 suitable carrier, by means of which they can easily be mounted and 



