A Vacuum Spectrometer. 7G9 



the natural size. C is the main conical bearing of the spec- 

 trometer — it carries on its upper end (not shown in PI. XVI. 

 fig. 1) the grating or prism -holder and near its lower end a 

 large divided circle D. Readings are taken on this circle 

 by means of micrometer microscopes MM, one division on 

 the head of which is one second of arc. Less accurate read- 

 ings may be rapidly obtained from the setting of the drum 

 N mounted on the axis of the worm W, which engages the 

 gear-wheel WW rigidly attached to the cone and divided 

 circle. The worm W is mounted on the bed-plate B, which 

 is bolted to the three legs of the instrument. The worm may 

 be thrown out of gear by a suitable mechanism. 



PL XVI. fig. 3 is an elevation of the upper part of the 

 instrument, showing a section of the evacuated region 

 and arrangement for leveling the grating or prism-holder II. 

 This holder may be centered by means of the screws c, and 

 it may be rotated about the axis of the cone C, which is 

 approximately coaxial with the cone C. 



PL XVI. fig. 2 shows a horizontal section of the instrument 

 through the axis of collimation; SO is the carrier of the slit 

 — this brass casting which carries the slit SS' is provided with 

 a window W of suitable transparent material and the necessary 

 conical plugs to admit of adjustment of the slit from the 

 outside as regards height, width, and position. The details 

 of this are not given as I have followed the construction 

 devised by Schumann. The slit carrier SC is held on the 

 collimator tube by means of the ground conical gearing- 

 provided with the screw-ring R x . The collimator tube is 

 provided with appropriate diaphragms, and is soldered with 

 the main bronze casting excentrically as shown in the figure. 

 M x and M 2 are concave mirrors, of silvered glass for the work 

 in the infra-red, and of speculum metal for ultra-violet work, 

 so placed as to render the beam falling on the grating 

 parallel, and then to bring the image of the slit on the 

 bolometer strip mounted in the cone BC. 



In case a prism be used instead of a grating the mirror M 2 

 is placed at M 2 ', and the bolometer case BO and the cap B'C 

 exchange positions. The details of construction of the carriers 

 of the mirrors M L and M 3 are given in the margin of fig. 3. 

 A diagram of the electrical connexions of the bolometer is 

 given in the margin of fig. 2. bi...b 2 represent the bolometer 

 strips 1-4, and 4 and 3 the balancing coils of manganin wire 

 which are bifilar wound and occupy the capsule c. . .c shown in 

 the bolometer case BO. For final adjustment of balance one 

 of the coils is shunted with the high resistance R'. 



A glass window is provided at the back of the bolometer 

 Phil. Mag. S. 6. Vol. 20 No. 118. Oct 1910. 3 E 



