THE AID OF THE ACHROMATIC FRINGES. 



109 



enlarged, respectively, while they remain in the field. It may be also done 

 (sometimes) by rotating M and M' in their own planes 180 (supposing the 

 mirrors to be of ordinary plate) and restoring the parallelism. 



Achromatic fringes, if well produced, are capable of enormous enlargement. 

 Thus I used a weak spectacle-glass (i diopter) with a very strong ocular, 

 obtaining a large telescope quite adequate for the purpose and enormous, 

 flawless fringes with ordinary plate glass in the interferometer. In such a 

 case, sunlight is to be used without a condenser. For the enlargement of the 

 field in the telescope a ground glass screen and sunlight will do very well. 

 In case of ordinary plate glass the fringes lie in a definite focal plane and 

 may lose clearness when displaced. 



108 



17L^ 



109 1 'I V..AA 



107 



81. Interferometry with the aid of secondary and tertiary achromatics 

 (satellites). I have described the occurrence of repetitions of the achromatic 

 fringes, on either side, at regular intervals, but with rapidly decreasing brill- 

 iance. In some adjustments as many as three distinct equidistant groups on 

 each side of the primary had been noticed. The secondary set is of very com- 

 mon occurrence and may even be sufficiently clear to be mistaken for the 

 primary group, unless a direct comparison is made. It is at first difficult to 

 surmise a reason for such a phenomenon in the case of white light. Some- 

 where in the train of apparatus there is a second and independent cause for 

 interference; i.e., a special path-difference, which, when superimposed on 

 either one or the other of the rays of the interferometer one or more times, 

 produces the phenomenon in question. 



I have since made some further investigations of these interesting fringes, 

 showing their availability in interferometry. A pair of identical half-silver 

 plates P, P', were prepared as before (fig. 108). These were then pressed 

 together on their silvered sides s, s', by steel clips, c, c r . The plates hold 

 between them what has been called a half-silvered air-film and thus offer 

 the requisite path-differences, increasing in regular steps, in accordance with 

 the number of reflection which occur within the film. 



These plates may now be placed anywhere normal to the rays, either at the 

 collimator or in front of the telescope, and it will be found that their presence 



