TRANSACTIONS OF THE SECTIONS. 



61 



In addition to the spirals that are traced, when the hand is wound first round the 

 apex of the cone descending to the base, by the pencil receding from the centre, on the 

 principle described ; spirals may also be traced by the pencil approaching the centre 

 from the extremity of the radius vector, when the pulleys are attached to the pencil 

 end of the horizontal bar, and the band is wound round the grooved cone ascend- 

 ing from the base. 



A A stand, with wheels moveable round the central point 0. 



B Horizontal bar, passing through the horizontal tube D on stand. 



C Tracing pencil, pressed down by vertical spiral spring. 



D Horizontal tube on stand. 



E Set of pulleys on stand. 



F Set of pulleys screwed on to the end of the horizontal bar. 



G Band wound about the grooved cone at the centre, and passing over pulleys at F and 2. 



H Handle attached to grooved cone, and held stationary with one hand ; while the stand 

 carrying horizontal bar, &c, is moved by means of a winch handle attached to the top of a 

 steel axis rising through the cone and handle. 



I The winch handle. 



On the Means of increasing the Angle of Binocular Instruments, in order to 

 obtain a Stereoscopic Effect in proportion to their Magnifying Power. 

 By A. Claud et, F.R.S. 



In a paper on the stereoscope, which Mr. Claudet read before the Society of 

 Arts in the year 1852, alluding to the reduction of the stereoscopic effect produced 

 by opera glasses on account of their magnifying power, he stated that, in order to 

 redress that defect, it would be necessary to increase proportionately the angle of the 

 two perspectives. This he proposed to do by adapting to the object-glasses two sets 

 of reflecting prisms, which by a greater separation given to the two lines of perspec- 

 tives, would reflect on the optic axes images taken at a greater angle than the angle 

 of natural vision. Such was the instrument that Mr. Claudet submitted to the 

 British Association, to prove, as he has always endeavoured to demonstrate in various 

 memoirs, that the binocular angle of stereoscopic pictures must be in proportion to 

 the ultimate size of the pictures on the retina, — larger than the natural angle when the 

 images are magnified, and smaller when they are diminished ; which, in fact, is 



