H8 C. Barus — Methods in Reversed and 



phenomenon superimposed on the first phenomenon. Roof- 

 shaped forms, fig. 12, strongly dotted, are also common, often 

 irregularly awned. 



Figure 13 may he consulted to further elucidate the subjects 

 under consideration. G is the grating, PP' the principal 

 plane of the objective of the telescope, a and b are two rays 

 interfering at the focus/', and leaving the grating parallel and 

 symmetrically placed to the axial ray of. The passage of the 

 coarse sagittate phenomena into the hairlike striations, as a 

 and b move farther apart, may then be accounted for in accord- 

 ance with the general theory of diffraction ; i. e. if the distance 

 apart of a and b is d and the principal focal distance of is B, 



A _ 2 



d ~ JR 



where z is the distance between the two fringes of wave- 

 length X. Hence z will increase as d decreases, agreeing with 

 the effect of fore and aft motion, or with the effect of simul- 

 taneous, large (2 - 5 cm ) displacement of both mirrors, neither of 

 which destroys the symmetry of the interfering rays. 



The motion of a single mirror, M or JY, for instance, does 

 destroy the symmetry, and it was shown in $8 that the limit- 

 ing range of displacement of -2o cm moves either a or b, •OVfr" 1 

 out of symmetry. The interferences thus vanish without 

 much changing in form or size, and vanish in all focal planes. 



The breadth of the blades of light act' and bb', figure 13, 

 capable of interfering is thus x on the grating and 



x cos 6 = -096 X -808 = -078 cm 



normally. Since the rays are parallel after leaving the colli- 

 mator, this would be about half the breadth of the effective 

 beam on the objective of this appurtenance. Thus 2 X "0776 

 _ ■i55«n^ increased by the width of the refracting edge of the 

 prism is the width of the strip of white light, which after 

 separation by the knife edge of the prism, furnishes the two 

 component beams which potentially interfere on recombination. 

 It is reasonable to suppose that the elements of these beams 

 come from a common source and that the width in question is 

 produced by the diffraction of the slit. 



This datum is more appropriately reduced to the angle at 

 the slit, a, within which the rays, capable of interfering with 

 each other after the interferometer cleavage, lie. As the colli- 

 mator used was I — 22 cm from slit to lens, 



a = 2cc COS 6/1 = -155/22 = •0070. 



Hence the angular width of the wedge of white light, with its 

 apex at the slit of the collimator and containing all the rays 

 which can mutually interfere, is about "007 radians, or less than 

 half a degree of arc. One would infer that a long (l) collimator 



