864 SUMMARY OF CURRENT RESEARCHES RELATING TO 



oil-glycerin-water immersion whicli will work througli 1/100 in. 

 covers, and with a balsam angle of not much less than 120° for best 

 results. An excellent and useful lens to add to the above series 

 would be a 1/5 in. (110° or 120° dry)." 



Right-angled Prism instead of a Plane Mirror.* — Mr. E. M. 



Nelson replies to Mr. G. Hunt's remarks f as follows : — " I can see 

 no possible advantage in going to the expense of a right-angled 

 prism, as in the commonest Microscopes I find the mirrors quite good 

 enough. One mirror I have gave me four or five images of the flame, 

 which would, of course, be fatal to good definition. This, however, 

 was corrected by turning the mirror round in its cell until a point 

 was found where all the images overlapped. Another mirror I have 

 is a concave, of about 10 ft, focus. I find no difiierence for ordinary 

 work. Any concavity in a plane mirror is bad, and ought to be 

 avoided, because it shortens the focus of the condenser, which will be 

 quite short enough, if it has any angle in it, without any further 

 shortening. 



I cannot say I can mention any definite object or object-glass in 

 which I could perceive any difference with mirror or lamp direct. If 

 any one is doing very special work, and fancies some error due to the 

 mirror, then turn it aside, and use the lamp flame direct. I cannot 

 see any advantage in a prism, which cannot possibly be so good as 

 nothing at all. One special advantage in using the lamp flame direct 

 is that one is not so liable to get the light out of centre. When a 

 mirror or prism is used, a slight touch, or shake of the table even, is 

 apt to throw it out of centre." 



Helot-Trouve apparatus for Electrical Illumination. | — Dr. H. 

 Van Heurck observes that the electric illumination of the Microscope, 

 hitherto little used, has just entered upon a new phase through the 

 new and thoroughly practical Trouve apparatus, which realizes all 

 that can be desired for the most difficult investigations in microscopy 

 and photo-microscopy. 



The battery consists of a small ebonite box, fig. 190, 15 cm. X 

 10 cm. X 18 cm., the inside of which is divided for two-thirds of its 

 height into six compartments, communicating at the bottom by a 

 small aperture between each. The elements, each consisting of two 

 rods of amalgamated zinc placed between three carbon rods, are 

 attached to the cover, being coupled in tension, and may be let down 

 into the liquid (potassium bichromate, sulphuric acid, and water) or 

 withdrawn therefrom, or more or less immersed according to the 

 power required at the time. 



The illuminating apparatus (fig. 191 in section), attached to the 

 front of the battery (fig. 190), or made to slide with universal joint on 

 a standard (fig. 192), so as to throw its light in any direction desired, 

 is the Helot-Trouve photophore, originally devised for surgical 

 operations and the examination of the cavities of the body. The 



* Engl. Mech., xli. (1885) p. 523. 

 t See this Journal, ante, p. 709. 



X Heurck, H. Van, 'Synopsis des Diatome'es de Belgique,' Texte, 1885, 

 pp. 219-22 (3 figs.). See also Journ. Soc. Arts, xxxiii. (1885) p. 1005. 



