CHAPTER I. 



AN INTERFERENTIAL CONTACT LEVER, WITH ACHROMATIC FRINGES. 



1. Apparatus. The method heretofore described for the measurement of 

 small angles by the aid of the rectangular interferometer, lends itself con- 

 veniently for the construction of apparatus like the contact lever or the spher- 

 ometer. Having in view work needing such instruments, I designed the fol- 

 lowing simple apparatus for the purpose : 



Figure i is a plan of the design ; figure 2 an elevation of the fork and appur- 

 tenances; figure 3 finally shows the same apparatus adapted for use as a 

 spherometer. The interferometer receives the white light from a collimator 

 at L. After the reflections and transmissions controlled by the mirrors M, 

 M f , N, N', and the auxiliary mirror mm', as indicated in the figure, the light 

 is conveyed into the telescope at T for observation of the interferences. The 

 mirror M' is on a micrometer with the screw s normal to its face. 



It is through the mirror mm' that the small angles are to be measured, and 

 this is therefore mounted at one end of the lever dc, capable of rotating around 

 the long vertical axle aa, in the circular fork FF. The latter is rigidly mounted 

 on the bed of the apparatus by aid of the stem / in the rear. The lever c is bent 

 upward at right angles at d, and it is here that the mirror mm' is firmly secured 

 by bolts, etc., as at . The spring k draws the lever toward the front of the 

 diagram, so that the blunt metal pin e suitably attached to the end of mm' 

 may be kept in contact with the glass plate g to be tested. 



The plate g, in order to be examined as to its degree of plane parallelism, 

 must be capable of sliding up and down, or right and left, under standard 

 conditions. To obtain these the stout bar G (rigidly attached like I to the base 

 of the apparatus) has been provided, carrying three set-screws h, h, h, the 

 points of which lie in the same circumference about 120 apart. They there- 

 fore constitute a kind of tripod against which the plate g is firmly pressed by 

 the flat spring or clip rr and screw *. This method of mounting may be 



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