CHAPTER VII. 



EXPERIMENTS MADE IN THE ENDEAVOR TO PLACE THE REVOLVING 

 MIRROR ON THE INTERFEROMETER. 



57. Apparatus. Revolving mirror normal. The ease with which the dis- 

 placement interferometer lends itself for the investigation of vibrating systems 

 induced me to make an attempt to combine the revolving mirror with the 

 interferometer, though this is obviously far more difficult, if at all feasible. 

 Figure 91 shows the first general plan, L being a beam of white light, N, N', 

 M, M' the mirrors of the quadratic interferometer, mm an auxiliary mirror 

 in the normal position. The telescope is at T. With the usual adjustment, it 

 is then easy to find the fringes, and any slight rotation of the mirror mm will 

 be registered by their displacement. 



If the mirror mm is at an angle, as at m'm' in the figure, and the rays are 

 reflected normally from a second auxiliary mirror nn, the fringes must again 

 appear if the adjustment is accurate. Moreover, any slight rotation of m'm' 

 will in tuin be registered by the displacement of fringes. 



If the mirror mm rotates, fringes will, therefore, appear for these two posi- 

 tions of the mirror mm. They will naturally flash through the field of vision, 

 but would be open to detection by the aid of a vibration or revolving telescope, 

 such as I used in case of vibrating systems. 



In the endeavor to carry out this difficult experiment, a powerful beam of 

 sunlight is to be introduced at L. To obtain this, I made use of a doublet of 

 two lenses, each about 10 cm. in diameter, respectively convex and concave, 

 and of a focal power of less than 2 diopters. By placing these at different 

 distances apart, focal distances of 10 to 20 meters were obtainable, admitting 

 of an easy guidance of the long beam, when the mirrors mm and nn were 7 

 to 10 meters apart. There was, consequently, always an overabundance of 

 light available. 



The difficulty with the experiment lies in the smallness of the image obtained 

 when the mirror nn is far off, as it must be, when measurements bearing on the 

 velocity of light are contemplated. Unless a special lens train is put in the 

 beam between mm and nn, this difficulty, so far as I see, is insuperable, inas- 

 much as the mirror mm, at least, can not be very large. An additional dif- 

 ficulty wa? encountered in the difference of the intensities of the light coming 

 from mm when normal, and from nn. It would, for this reason, hardly be 

 practicable to compare the former fringes with the latter as to relative dis- 

 placement, the plan I had in view. But the normal reflection from mm may be 

 blotted out by screens m lt m\, placed as shewn in the figure, since this part of 

 the mirror is not used in connection with nn. Moreover, the displacement of 

 fringes due to the time lost in passage of light from mm to nn and return may 

 be compensated by the micrometric rotation of nn around a vertical axis, as 



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