the Reflective Goniometer. 285 



crystal is secured, and then adjusted by making the vice re- 

 volve round one or the other of its two axes, making right 

 angles with one another. Also the distance between the 

 faces under observation may be greater than the aperture of 

 the small telescope. The difficulty arising from this circum- 

 stance may be overcome by interposing a plate of glass 

 mounted so as to be adjustable in azimuth and zenith-distance, 

 supported by a stem secured to the under plate of the instru- 

 ment. The observation is made by bringing the image of A 

 seen by reflexion in the face of the crystal into coincidence 

 with the image of the signal B seen by reflexion in the vertical 

 plate of glass. 



A small telescope having a power of about 3 may be applied, 

 either for the purpose of observing with greater precision the 

 coincidence of the image of A, as seen by reflexion in a face 

 of the crystal, with the signal B or its image, or of bisecting 

 the reflected image of signal A with the vertical spider line. 

 The telescope is attached to an upright support, the foot of 

 which is fastened to the upper surface of a strip of brass bent 

 twice at right angles in opposite directions, the upper surface 

 of the brass plate being in the plane of the upper surface of 

 the graduated circle, and secured by a screw to the under 

 surface of the base-plate. 



By attaching the upright stem of the telescope by one of 

 the screws to the graduated circle, the telescope becomes avail- 

 able for measuring the minimum deviation of a ray of any 

 colour through two inclined faces of a crystal. 



The best bright signal is the light of the sun reflected from 

 the mirror of a heliostat through a slit so adjustable that the 

 middle of it remains fixed in space while its diameter admits 

 of being varied. An image of the sun in the focus of a lens 

 of about 30 millims. focal length, formed by the light reflected 

 from a plane mirror, is sufficient in most cases. Much inferior 

 to these is the light of a lamp or that of the sky reflected by 

 a plane mirror through a small opening in a screen. It is hardly 

 necessary to remark that perfect distinctness of vision of the 

 signals, which should be equidistant from the centre of the 

 instrument or very nearly so, is essential to accuracy. When 

 the eye of the observer is not adapted to the distance of the 

 signals, the use of a Galileo's telescope of low power will 

 greatly increase the accuracy of the result. 



