TRANSACTIONS OF THE SECTIONS. 23 



As the reflectors are onlj' half the length of achromatic refractors of equal 

 aperture, aud their price is scarcelj^ a quarter that of refractors of the same power, 

 I believe, wheu their qualities are more generally known, they will he very ex- 

 tensively employed. 



On a Neiu Anemometer. By L. Casella. 



On a Variable Diaphragm for Telescopes and Pliotographic Lenses. 

 By A. Clatoet, F.B.S. 



This diaphragm is fonned by a number of narrow strips of india-rubber, the two 

 ends of which are respectively attached to two short tubes placed near the object- 

 glass in the telescope, or in the middle of the photographic optical combination. 

 One of the tubes can, to the extent of ISO^, be made to revolve by the handle of an 

 endless screw placed outside, aud acting cui a half-toothed cii'cle fixed on the 

 moveable tube, so that one end of each strip being attached to the immoveable 

 tube, and the other end being carried round as much as half its circumference by 

 the circle to which it is fixed ; each strip, extending sufiiciently by its elasticity 

 during that motion, hides more and more the aperture of the tube, until it coin- 

 cides entirely with its diameter. All the strips, acting simultaneously in the same 

 manner, cross each other, aud iu doing so gradually reduce the aperture of the 

 tube imtil it is shut entirely, wheu the strips overlapping one another form two 

 cones with their apices turned towards each other. 



The action of this diaphragm, being as rapid as may be needed, is very effective. 

 It may be usefully employed in astronomical investigations, allowing the observer 

 to alter gi'adually, or at once, the apertm-e of the lens, according to circumstances 

 and different kinds of experiments. 



This diaphragm is also very convenient in photogi-aphic operations, as it enables 

 the artist, dming the sitting, to increase or reduce rapidly the aperture of the lens 

 according to sudden changes in the intensity of light. But it has another most 

 important advantage in taking portraits, which consists in affording the means, 

 first, to impart to the portrait a certain degree of softness resulting from the 

 spherical aberration produced by the whole aperture, and immediately after, by 

 gradually reducing the apertm-e until it is shut entirely, to communicate to the 

 picture upon the slightly confused image another one sharply defined ; the blend- 

 ing of the two effects producing an harmonious and artistic poi-trait, which could 

 not be obtained separately, either by the whole aperture, or by the centre of the 

 lens only. 



Omi Magnifying Stereoscope with a Single Lens. By A. Ceattbet, F.E.S. 



In the ordinary stereoscope each eye has its lens, but if a single lens be placed 

 on any point where the optic axes may be directed and cross each other, such a lens 

 is sufhcient to enable both eyes at once, and each separately, to see through it, the 

 slide placed behind containing the two pictures ; each eye perceiving only the 

 one, the perspective of which belongs to it. In looking with the two eyes the 

 combined image appears in relief, and just as if it were situate on the lens itself, 

 where, by the requked convergence of the two optic axes, the two pictures seem 

 really to coincide. 



The pictm-es are mounted as usual on a slide, one near the other, but with the 

 right perspective on the left, fmd the left perspective on the right, from whence 

 through the single lens they can only, of course, reach their respective eyes. The 

 slide must be placed at the distance behind the lens, which, corresponding with 

 its focus, gives a distinct vision of the picture. The observer must place himself 

 exactlj- before the lens, which is to be precisely in the middle of the space between 

 the ej-cs and the pictures. Placing against the lens and in front of it a black 

 card with a square opening, the picture appears on the plane of that opening, and 

 as if it were seen on its usual square mounting. 



For persons who (converging the optic eyes upon the lens) can keep them in 

 that position while they look thi'ough it at the pictures behind (which, by a little 

 practice, is easily acquired), the stereoscopic illusion is very complete, and the 



