26 



THE INTERFEROMETRY OF 



passing the optimum, vanished as inflated arrows. The range of visibility 

 was, as before, about 3.5 mm., so that the change of order has not had any 

 further marked effect, such as might be anticipated. As in the preceding 

 paragraph, if the impinging collimated beam is narrowed, the range of visi- 

 bility decreases ; in fact, the arrows themselves are reduced to slightly oblique 

 lines. Within the limits given the fringes are well adapted for interferometry. 



First-order spectra are not available because of the large value of i' in the 

 case of the right-angled prism. 



Taking the results of the last two paragraphs together, the increase of the 

 range of displacement is due to the dispersion of the prism. The breadth of 

 the pencil, diffracted at the slit, after leaving the collimator and prism, 



12 



13 



increases. It was shown in the earlier report that inversion of spectra on a 

 longitudinal axis does not preclude the possibility of interference. Taken as 

 a whole, therefore, the present results have a direct bearing on Huyghens's 

 principle. 



14. Prism methods without grating. A more interesting method, in some 

 respects, in which the grating is entirely dispensed with, is shown in figure 

 14. L is the beam of white light from a collimator, P a refracting prism (here 

 with a 60 prism angle), M and N the opaque mirrors, with either or both 

 on a micrometer, P' a silvered reflecting prism (here right-angled). The 

 telescope is at T and should have high magnification. The rays L are refracted 

 into a b c and a' b' c' and the two spectra observed by the telescope at T. 

 Each of the prisms should be on 3 adjustment screws, as well as the mirrors. 

 P must be revolvable slightly around a vertical axis and capable of fore-and- 

 aft motion. P' is preferably a large prism placed on a tablet. The rays b and 

 b' are made collinear before P' is inserted, and both the rays c and c' must 

 come from near its edge. 



