ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 755 



Focusing Magnifier.* — This instrument (fig. 177), made by Taylor, 

 Taylor and Hobson, of Leicester, is intended for examining the definition 

 of an image on the camera screen, and is arranged to close like a tele- 



Fig. 177. 



scope for compactness. The screw-ring forms an adjustable stop to 

 limit the withdrawal of the eye-piece to suit the sight of the user. 



Leadbhater, L. — Photographing Crystals. 



[Lecture atRotherham Photographic Society.] 



English Mechanic, lxxxii. (1905) pp. 152-3. 



Marktanner-Turneretscher, d. — Wichtigere Fortschritte auf dem Gebiete 

 der Mikrophotographie und des projektionswesens. 

 Separat-Abdraok aus Jahrbuch f. Photog. und Reproduktionstechnik f. das Jahr 

 1905. HaUe a. S., Wilhelm Knapp. 



(5) Microscopical Optics and Manipulation. 



Braun's Methods of Identifying Sub-microscopic Structures 

 Allied Investigations on Double Refraction.f — F. Braun has found 

 that certain substances — e.g. electrically pulverised metals, produce a 

 grating-like structure when viewed with polarised light. This effect is 

 in full agreement with the electro-magnetic theory of light. It has 

 also been found that certain organic substances specially treated with 

 gold solutions give similar effects. Hence it would seem that, either 

 the finely-divided gold, or some compound of the gold and the organic 

 substance, must be anisotropic. Braun's experiments were all made 

 with transparent light, but similar results have now been attained with 

 reflected light. It appears from these later experiments that the light 

 which vibrates parallel to the grating-bars is reflected more intensely 

 than that in the perpendicular direction. This, again, is in accordance 

 with theory and with the behaviour (only reversed) of the transparent 

 light. The method of observation is to place the object on the stage 

 in the usual way, and to arrange above it a cover-glass inclined at 45° 

 to the horizontal. The plane is set horizontally, and polarised light is 

 then made to impinge on the cover-glass ; it is then reflected down- 

 wards through the object to the mirror ; is again reflected, and passes 



* Catalogue. 1905, p. 23. 



t Ceutral-Zeit. f. Opt. u. Mech., xxvi. (1905) p. 188. 



3 E 2 



