386 Lord Blythswood and Dr. Marcliant on 



moving the prisms) until the required line appears in the field. 

 In order to do this easily a long rod was attached to the 

 handle H, which could be turned by the observer at the eye- 

 piece: this rod appears in the photograph together with a 

 similar rod for opening and closing the slit S 2 . 



The condensing-lens C is used to focus the source of light 

 (shown between the poles of a magnet) on the slit S x . All 

 these adjustments are made with the echelon out of position. 

 In order to obtain the best possible results with the instrument, 

 the slits S|, S 2 should both be set parallel to the steps of the 

 echelon. The easiest way, of course, of doing this is to make 

 both the slits and the edges of the echelon as nearly vertical 

 as possible. The instrument is, however, nothing like so 

 sensitive to this adjustment as the concave gratings with 

 14,000 or more lines to the inch. 



When the adjustment is complete and the particular line 

 wdn'ch it is required to examine is focussed on the slit S 2 , the 

 echelon may be placed in the path of the beam. It should be 

 noticed that the focus of the orders when the echelon is in 

 position is not necessarily the same as that for the slit S 2 

 when the echelon is removed. In this particular instrument, 

 owing to some error in the plates, the focus was obtained 

 when the eyepiece was about an inch further out than it was 

 with the slit S 2 sharply focussed, with the echelon removed. 



Jn the mounting used here a form of parallel motion was 

 adopted for moving the echelon in and out of position ; in 

 setting, therefore, the echelon was moved across the field until 

 the brightness of the lines seen in the eyepiece was a maximum. 

 When once this position had been determined, marks were 

 made on the stand and the echelon-mount, so that the grating 

 could always be placed in that position. 



There was one other motion of which the grating was 

 capable, viz., a rotation round a vertical axis in the centre of 

 its stand: this motion could be effected by a screw, with a 

 counter spring for obtaining the motion in both directions. 

 The use of this adjustment is explained later. Finally, the 

 echelon was placed on a stand with levelling-screws, so that 

 the edges of the plates could be set exactly vertical. (See 

 above.) 



§ 3. Constant of tlie Grating. 



As stated above, the grating was made of a light flint-glass, 

 the refractive index of which was given by the Jena manu- 

 facturers for five principal wave-lengths. From three of these 

 values the values of the constants in Cauchy's expression for 

 the refractive index were worked out ; the resulting equation 



