( 732 ) 



silvered on the back side. ^) The angle of I amounts to 30°40' ; the 

 plane of' entrance V ■ — 52 X -52 nun. lai-ge — forms an angle of 

 about 40' with the wave-front, which causes the disturbing reflex 

 to be thrown aside. The object-glass, consisting of three lenses, has a 

 diameter of 67 mm., a focal distance of 260 mm., so that the a[)er- 

 ture amounts to ^ j ^ . The lateral spherical aberration is according to 

 calculation of the order 0,01 mm. A small part of the convex front 

 surface has been blackened to i)i'event reflection ; square diaphragms 

 D have been placed in the collimator tube for the same pui'pose. 

 The whole system of prisms is {)laced under a closed metal cover 

 K ; inside this the necessary chemical substances are supjdied in order 

 to protect the sensiti\'e glass surfaces against the action of Avater- 

 vapour, carbonic acid, hydrogen sulphide etc. Whether these measures 

 will prove effectual remains, as a matter of course, to be seen after 

 a considerable lapse of time. 



At the end of the collimator tube the "slit holder" is arranged so 

 as to rotate round its axis. The bilateral entrance slit I, which is 

 provided with a prism for comparison, is 3.5 mm. long, it is slightly 

 curved (radius of curvature 70 mm.), and can rotate a little so that 

 the slope and curvature of the spectral lines is compensated for a 

 mean colour; the slit is focussed by means of a spiral groove. A 

 mirror silvered at the front side directs the rays towards the lens, 

 which on their way back pass along its upper or lower side so as 

 to reach the exit-slit U ; the latter is also bilateral, 3,5 mm. long, but 

 rectilinear. It may be exchanged for monocentric non-reflecting ej'e- 

 pieces with a focal distance of 9 or 25 mm., or for a normal camera 

 60 X 90 mm., by means of which ojjIv a small spectral region can 

 of course be photographed at the same time. 



The whole apparatus is constructed without anv iron, and mounted 

 very compactly on a marble slab. The adjusting screws form a right- 

 angled triangle, one of the catheti lying under the optical axis, whose 

 height above the plane of the table is 125 mm. 



For measurements in the ultraviolet the object-glass is replaced by 

 a quartz-tluorite achromatic lens (y = 33 mm., /'=260mm., apert. 

 Vg) ; it would of course be too expensive to fill the whole aperture; 

 a couple of quartz-half prisms according to Cornu is also provided. 



') In many respects it may be preferable to fix a melal mirror with glycerin 

 to the back plane, for it is easy to remove it, and also to adapt the apparatus 

 for transmitted light ; in this case a telescope or a spectrograph with camera has 

 to he added ; the same mirror may be used, if necessary, to give the desired direc- 

 tion to the light. Besides, some alloys reflect considerably belter than silver in the 

 ultraviolet (about 3:20 (ift). 



