342 



BEPOET — 1888. 



telescope atff, focussed for infinitely distant objects, reveals a system of interference 

 bands, whose direction should he vertical, if the adjustments could he perfectly 

 carried out in the manner intended. 



The success of the method does not require the complete symmetry of the 

 diagram. If the reflections at d, e are effected by a right-angled prism, it is neces- 

 sary that cd, fe be parallel to one another, but not that they be parallel to the 

 surface b. Supposing all the surfaces to remain vertical in any case, the posi- 

 tions of b, f, and the incident ray ab may be chosen arbitrarily. If the distance de 

 between the parallel courses is not closely prescribed, one adjustment by rotation 

 of the mirror c will suffice. In my experiments the optical parts were mounted 

 upon a large iron plate, so that the movable pieces c, de could be shifted without 

 loss of level. The incident ray ab was defined by a small hole near the paraffin 

 lamp which served as a source of light, and by the centre of a moderately large 

 circular aperture perforated in a screen and illuminated when necessary with a 

 candle. The mirror c was then rotated until the vnys cd,fe were parallel. This 

 was tested by observing the equahty of their mutual distances near the extremities 

 of their course. 



If the distance between the parallel rays is prescribed, the adjustment is more 

 troublesome. The line fe being fixed, sights are laid down defining the desired 

 position of cd. These sights, as well as those before referred to defining the inci- 

 dent ray, have now to be brought to apparent superposition as seen by an eye 

 looking along dc. For this purpose t7co conditions have to be satisfied by, and two 

 motions must be provided for, the mirror c. One of these should be a movement 

 of rotation, and the other of translation in a direction nearly perpendicular to the 

 plane of the mirror. Thus the mounting may consist of a circular turntable resting 

 upon the iron plate, the curved edge of which is guided by the sides of a V, cut out 

 of a flat piece of metal and clamped to the plate. In each position of the V the 

 angular motions are easily swept over, and the double adjustment is effected with- 

 out much difficulty. AVhen the parallelism of the rays is secured, the insertion of 

 the reflecting prism is all that remams. The adjustment of this is best eflected 

 with the eye at the observing telescope, which at this stage should be focussed 

 upon the small aperture in the neighbourhood of the flame. By a motion of the 

 prism parallel to its hypothenuse the two images are brought to coincidence,' and 

 then the bands appear, if not at once, when the telescope is accommodated for 

 infinitely distant objects. 



• It should be noticed that if the object were at infinity, or if with the aid of a 

 collimating lens an image of it were thrown to infinitj-, the two images as seen 

 focussed through the telescope would overlap in any case ; for it may be proved that, 

 whatever may be the positions of the five retiecting surfaces, the two emergent rays, 

 corresponding to any incident ray, are necessarily parallel. 



