18 



Mr. A. A. Michelson on the Application of 



a rod (as shown in the Plate), so as to be controlled by the 

 observer, and the distance between the slits was read off by a 

 small telescope beside the eyepiece of the large one. 



This simple and effective device has but one disadvantage. 

 When the angle to be measured is so small as to be just 

 beyond the power of the telescope, it is necessary, in order to 

 observe the first disappearance of the fringes, that the slits 

 should be separated to the full width of the aperture. Under 

 these circumstances the two pencils meet at a very large 

 angle, usually several degrees, and the corresponding distance 

 between the inteference-fringes is but a few thousandths of a 

 millimetre, and in order to be visible as such must be highly 

 magnified and accordingly very faint. 



This difficulty may be entirely overcome by using instead 

 of the telescope one of the forms of refractometers shown in 

 figs. 10 and 11. 



Fig. 10. 



Fig-. 11. 



recognized 



as 



one of the forms 



The first of these will be 

 already described *. 



The apparatus being adjusted so that interference-fringes 

 are visible, the telescope (a comparatively small one) is 

 adjusted until the two images of its cross-hairs in the mirror 

 m coincide with the cross-hairs themselves. 



When this is the case, it is clear that when the whole 



apparatus is pointed at a star (the mirror m being now 



* " Measurement by Light- Waves," by A. A. Michelson. 

 Journal of Science, xxxix. Feb. 1890. 



American 



