142 



N0VELTIE8. 



brougM into view in succession, and can'thus be named from the 

 written catalogue. 



M. MoUer also prepares a test slide, containing twenty examples 

 of diatoms, arranged in a straight line, and similarly mounted to 

 the one described. 



The idea of type slides is evidently a very good and useful one. 

 It would be far more convenient if typical specimens could thus be 

 mounted for reference, instead of being compelled to keep a sepa- 

 rate slide for each. 



Immersion Lenses. — The method of examining objects with 

 the aid of a drop of fluid placed between the anterior combination 

 of the objective, and the glass cover of the slide, has hitherto 

 found more favour with continental microscopists than those of our 

 own country. Foreign opticians have carried out the principle most 

 successfully, and we have seen some excellent results produced by 

 object glasses on this principle, costing comparatively a low price. 

 Recently, however, Messrs. Powell and Lealand have turned 

 their attention to the subject, and have succeeded in their efforts ; 

 we have had an opportunity of testing the arrangement with a 

 -^ objective, and have found the power of definition of the latter much 

 increased. The test object was Navicula rhomboides, — and a very 

 delicate specimen of it, which has always required some trouble to re- 

 solve. It was first viewed by the ^th in question, in the ordinary way, 

 and the two sets of markings were just made out; the new adjust- 

 ment was then added, a drop of fluid placed on the front lens, and 

 the illumination and object left untouched. When brought into 

 focus the lines became distinct and unmistakable dots. 



It will be observed that the original form of the object glass re- 

 mains unchanged. The new system involves the addition of 

 a front lens, which is removed when the object has to be examined 

 in the ordinary manner. 



New Pseudo-Binocular Prism. — At the October meeting, 

 Mr. Henry Crouch, of 54, London "Wall, exhibited a modifi- 

 cation of Wenham's prism, which, by a slight alteration of adjust- 

 ment, presents a pseudoscopic, or inverted image. As is well 

 known, an ordinary Wenham prism has four surfaces, the relative 

 position of the two reflecting surfaces determining the angle of 

 the emergent ray. The base, as ordinarily mounted, occupies half 

 the field; the left half of the cone of rays being transmitted 



