520 BEPOKT— 1886. 



mercury whicli works the pump from contact with the external atmosphere. When 

 the Sprengel pump is used for producing and maintaining a nearly complete vacuum 

 during a course of experimenting which lasts over a good many days or weeks, it is 

 found that the mercury running down through the pump and discharging into the 

 open air takes up minute quantities of air, which it carries with it in ^ state of very 

 intimate mixture. This intimately mixed air the mercury does not lose in passing 

 through the air-traps, hut the air is deposited along the walls of the pump head, 

 and collects into bubhles which are at first exceedingly minute, and gradually in- 

 crease to a sensible size. These bubbles ultimately escape from contact with the 

 glass and deteriorate the vacuum. 



The pump described consists of a combination of a single fall Sprengel with a 

 Geisler pump, and in addition the movable reservoir is furnished with an air-tight 

 stopcock, by means of which the air is got rid of from that reservoir and prevented 

 from re-entering. 



In using the pump it is first worked as an ordinary Geisler pump till nearly all 

 the air of the enclosure to be exhausted has been removed. In this way advantage 

 is taken of the comparatively great speed of the Geisler pump. When a good 

 exhaustion has been obtained the stopcocks and the lift of the movable reservoir 

 are so managed that the pump is thereafter used as a Sprengel — the small remain- 

 ing traces of air being pumped by the Sprengel into the exhausted chamber of the 

 Geisler ; and when a sufficient quantity of air has been collected in this chamber 

 it can be removed by a single Geisler operation. 



5. On the Cutting of Polarising Prisms. Sy Professor Silvanus P. 



Thompson, D.Sc. 



The author described a method of cutting Nicol prisms analogous to that 

 communicated in 1885, the end faces being inclined at 70° to a natural face of 

 the crystal, and having a line of mutual intersection at right angles to the natural 

 long edges. The author also described a new method devised by Mr. Ahrens of 

 cutting the triple polarising prism described by the author at the Aberdeen meeting. 

 The new method of cutting effects a great saving of spar, and depends upon finding 

 a certain characteristic plane in the crystal which defines the external longitudinal 

 faces of the prisms. 



6. On a Varying Gylindrical Lens. By Tempest Andekson, M.D., B.Sc. 



A cylindrical lens of continuously varying power lias long been a desideratum, 

 and one was constructed and described by Professor Stokes, Pres. R.S., at page 10 

 of the Report of the British Association for 1849. He points out that — 



' If two piano-cylindrical lenses of equal radius, one concave and the other 

 convex, be fixed one in the lid and the other in the body of a small round box with 

 a hole in the top and bottom, so as to be as nearly as possible in contact, the lenses 

 will neutralise one another wlien the axes of the surfaces are parallel, and by merely 

 turning the lid round an astigmatic lens may be formed, of a power varying con- 

 tinuously from zero to twice the astigmatic power of either lens.' 



This very beautiful optical contrivance has the disadvantage that the refraction 

 varies from zero in both du-ections at once, the refraction at any given position of 

 the lenses being positive in one meridian, and negative or concave to an equal degree 

 in a meridian at right angles to the first ; moreover, there is no fixed axis in which 

 the refraction is either zero or any other constant amount. It has in consequence 

 never come into extensive use in the determination of the degree of astigmatism. 



The author has planned a cylindrical lens in which the axis remains constant in 

 direction and amount of refraction, while the refraction in the meridian at right 

 angles to this varies continuously. 



A cone may be regarded as a succession of cylinders of different diameters 

 graduating into one another by exceedingly small steps, so that if a short enough 

 portion be considered, its curvature at any point may be regarded as cylindrical. 



A lens with one side plane and the other ground on a conical tool is therefore 



