292 report — 1879. 



slowly, as you perceive the cover is placed inside the camera, and when in perfect 

 order may be worked noiselessly. 



Note. — Can be kept open any length of time without exertion. Can be closed 

 instantaneously. No vibration. 



14. On a Binocular Spectroscope. By G. Johnstone Stonet, M.A., F.B.S. 



In this instrument, which is not yet finished, there will he two collimators placed 

 parallel to one another, provided with horizontal slits which lie in the same right 

 line. The telescopes will also he parallel, and will he in a convenient inclined 

 position. Between them lie the prisms, which are a semiprism, a complete prism, 

 and another semiprism of bisulphide of carbon, plunged in a tank of water, and 

 connected by Mr. Grubb's simple automatic motion, which consists of a link and 

 two cogged wheels. The angles of the prisms will be large, their faces disks of 

 optical glass five inches square, and the collimators and telescopes are of two-inch 

 aperture. With this instrument the spectrum will be spread out vertically, and 

 the lines in it will be horizontal. 



15. On a Simple two-prism Automatic Motion. 

 By G. Johnstone Stoney, M.A., F.B.S. 



In this arrangement, which is easily constructed by an amateur, and which has 

 worked for a long time satisfactorily, the collimator and telescope are fixed on two 

 boards, A and B, jointed together by a hinge, the prisms stand on little wooden 

 tables, a and b, which turn on pivots fixed in the boards, A and B, under the 

 middles of the first and last surfaces of the prisms respectively. The motion of 

 each of these tables is trammelled by a link, connecting a projection from the front 

 of tbe table with a point on the front of the board upon which it does not stand, 

 the radius from the first end of the link to the pivot on which the little table turns 

 bein" four times the radius from the other end of the link to the hinge of the 

 boards, and these radii being parallel to one another when the instrument is set 

 upon the middle of the spectrum. Thus the table a is connected by its link with 

 the front of the board B, and the table b with the board A. Strips of brass are 

 let into the wood to receive the holes in which the pivots and links work, and the 

 links are also of brass. One of the links is arched upwards and the other down- 

 wards, sufficiently to clear each other. Pencilled lines are drawn on the little 

 tables to mark the proper positions of the prisms, so that they can be replaced 

 without delay after they have been removed. 



16. On Scales of Variable Length for the eye-pieces of Spectroscopes. 

 By G. Johnstone Stonet, M.A.,F.B.S. 



This communication was made chiefly for the purpose of inviting the attention of 

 instrument makers to the want of a convenient scale in the field of view of a 

 spectroscope, the interval between the divisions of which can be varied to suit the 

 part of the spectrum under examination. It was suggested that the property of a 

 spiral spring, to retain the equal spacing of its spires when extended, might be 

 made use of. A scale, of which the length can be varied, is also very much wanted 

 for use with maps of spectra. Possibly a scale simply laid down on vulcanised 

 india-rubber would work sufficiently well here. 



17. On Flight and its Imitations. By F. W. Breary. 



