INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 867 



glass, a ring is spun, whicli may be made as narrow as is desired, 

 and with its rounded top extending above the covering glass. 



The slides are laid aside in a dry place for at least a week to 

 harden, when the superfluous varnish can be cleaned off from the 

 glass with a bit of soft linen rag and rottenstone and water, rubbing 

 the whole mount gently with circular strokes. This removes the 

 superfluous varnish from the glass to the edge of the ring, leaving 

 it with a clean circular edge, and at the same time rubs down any 

 inequalities which may exist in the ring itself. After this, wash the 

 slide well in fresh water with a soft brush to remove all traces of the 

 rottenstone, and gently dry it with a soft cambric handkerchief. 

 When it is dry, a few circular strokes with dry cambric on the end 

 of the finger, will give the ring a semi-polish, which leaves it with a 

 very neat finish. 



The whole slide is usually cleaned with the rottenstone and 

 water, so that when it is dried and gently wiped, it is ready to receive 

 the label. The whole process is quite expeditious, and the results 

 are so satisfactory, in the permanence and finish of the slides, that 

 the author is confident, if any one gives it a fair trial, it will super- 

 sede all other cements for a like purpose. 



Novel Form of Lens.* — Dr. Cusco, ophthalmic surgeon in one 

 of the hospitals of Paris, has invented a lens of variable focus, in 

 which the pressure of a column of water or other transparent liquid 

 is made to alter the curvature of the flat faces of a cylindrical cell of 

 brass closed with thin glass disks. The pressure can be regulated 

 by a manometer gauge to any required degree within the limits of 

 working. It is said that the lens gives a sharp, well-defined focus. 

 It was constructed for Dr. Cusco by M. Laurent. 



Swift's Radial Traversing Substage Illuminator.— Messrs. Swift 

 have further developed the idea of a " swinging substage " by the 

 apparatus shown in Figs. 82 and 83. The essential feature is the 

 addition of a second sector with condenser at right angles to the first. 



The following is their description of the apparatus (slightly 

 abridged). 



" This apparatus has been constructed for the purpose of increas- 

 ing the resolving px-ojierty of high-power objectives by causing still 

 more oblique pencils to impinge on the object than can be obtained by 

 any other method. The arrangement consists firstly of an arc-piece 

 fixed below tlio stage radial to an imaginary line drawn through tlic 

 axis of the objective, and in tlie same plane with the object. On this 

 an achromatic condenser of special construction is made to travel, 

 thus keeping the rays of light on the object during its entire travers- 

 ing, these rays converging in a focus through the front lens in a highly 

 concentrated form. The condenser is illuminated by a rectangular 

 prism. 



The next part of the contrivance consists of a second arc-picco 

 placed at riglit angles to the former one ; this also cari'ies a similar 

 achromatic condenser and illuminating prism, and moves radial to 



♦ 'Nntnr.-.' xxii. (]»s()) p. 2S0. 



3 I. 2 



