1886.] Light reflected at nearly Perpendicular Incidence. 



281 



lines DE, FE. A levelled slab of glass is provided, on which to rest 

 the feet of the stand carrying E. The mirror is now brought into a 

 vertical plane, and may then be shifted on the slab without loss of 

 this adjustment. The remaining double adjustment is best made 

 systematically. By rotation about any vertical axis, the central ray 

 may be caused to pass over one of the needles. If it fails to pass over 

 the other, the axis of rotation must be shifted backwards and for- 

 wards until a suitable rotation allows satisfaction of both conditions. 

 The ray now follows in both cases the course FGH, and the mirror 

 H, with the sharp edge, may next be pushed in so as just to catch the 

 ray in question and send it to the observing telescope (half of a small 

 opera glass) at I. 



The adjustments for the auxiliary light on the left hand side are a 

 simpler matter. All the mirrors being levelled, the central ray is 

 brought to the point H', in the prolongation of IH. Nothing then 

 remains but to turn the final (vertical) mirror round H' until the re- 

 flected ray coincides with HI. When the eye looks in along this 

 line, the bright spot should be seen in the same position from both 

 mirrors. 



To guard against accidental displacements, the movable pieces were 

 usually secured with a little sealing wax. A diaphragm at K limits 

 the field of view, and is so placed that the aperture is bisected by the 

 division line H. It is not necessary to do more than allude to various 

 screens employed to cut off stray light and render the room as dark 

 as possible. 



The principal trouble experienced, that of making and retaining 

 the adjustments, is connected with the rather large scale of the appa- 

 ratus, which made it difficult to use a single levelled bed for all the 

 movable pieces. The question is thus suggested, what is it that fixes 

 the absolute scale ? And the rather unexpected answer must be — the 

 diameter of the pupil of the eye, which is the only linear quantity 

 concerned.* 



In order to understand this it is necessary to bear in mind that 

 although in describing the adjustments we speak of a single ray only, 

 we are of necessity really dealing with a complete beam. The obser- 

 vation of a match requires that the two parts of the field of view 

 have finite angular magnitudes, and from every point of the field 

 there must proceed a pencil of rays limited by the pupil, or by the 

 telescope. If all these rays are to be treated as sensibly parallel 

 during their passage through the apparatus, certain limitations must 

 be observed. For easy observation the field of view should subtend 

 at the eye an angle of not less than a degree, so that if no telescope 

 be employed the defect of parallelism must exceed this amount. 

 The linear scale of the apparatus is not thus fixed, however, for we 

 * The wave-length of light may be regarded here as infinitely small. 



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