﻿350 On Fixed- Aim Spectroscopes. 



were adjusted, it is only necessary either to turn each one in 

 a measured amount, or, more simply, insert a thin piece of 

 glass or metal between the screws on each side and the prism. 

 A still better plan, if many prisms are to be used, is to have 

 each prism mounted on a base of the required size on which 

 it has been adjusted once for all and then cemented. The 

 adjustment, as will be shown hereafter, does not need to be 

 particularly accurate. 



The mirror and prism having been adjusted to each other 

 and to the spectrometer axis once for all, the other adjust- 

 ments preliminary to the use of the instrument are very 

 simple. The first operation is to make the axes of the colli- 

 mating and observing telescopes parallel. This is most simply 

 done by removing the prism-mirror system and allowing the 

 beam for the collimator to fall directly on the objective of the 

 view-telescope. Since the constant lateral displacement of 

 the beam by the system will be 2a, it follows that if the 

 aperture of the telescope is, say, 2Ja, sufficient light will fall 

 upon the second objective to form a distinct image of the slit. 

 Since, however, the use of such a small portion of the objective 

 is apt to cause a lateral shifting of the image, it is better 

 either to mount the view-telescope on a short cross slide, or 

 on a rocking arm, the motion of which is accurately rectili- 

 near, or, what is better, to use a small double-reflection prism, 

 fig. 6 b (PI. IX.), which will laterally deflect the beam by the 

 amount required. If a system like that in fig. 7 (PI. IX.) is 

 used, the simplest method of initially setting the direct image 

 on the cross wire of the observing telescope is to set up at the 

 side of the slit and at an accurately measured distance 2a 

 from its centre, a second slit or fine point, and bring the 

 image of this to the centre of the field. 



The second step is to determine the zero of graduation, 

 which is done by replacing the optical system and revolving 

 the mirror until it is perpendicular to the line of collimation. 

 With the first form of instrument this is accomplished simply 

 by bringing the image of the slit into coincidence with the 

 slit itself ; in the second form it is most conveniently done 

 by setting up a point at a distance from the mirror equal to 

 the principal focal distance, bringing this point into the line 

 b m s by removing the eyepiece and sighting with the eye past 

 the cross wire at the distant point s, and then revolving the 

 mirror until the image of this illuminated point coincides 

 with the point itself. The reading of the circle correspond- 

 ing to this position of the mirror is called the zero reading. 

 The prism is then replaced in position by being brought 

 against the screws e, /, g, A, and the whole system revolved 



