190 * SEC. 7. LIGHT. 



making improvements which greatly facilitate the use of the instrument, by 

 a change in the construction of the wedges, and have thus reduced the irregu- 

 larities frequently observed in the polarisation of bodies of low specific 

 gravities to from one to two tenths of a per cent, in each part of the scale. 

 They have thereby done away with the principal cause of the variations 

 which so frequently occur in the observations of different analysts. 



875. Jellett & Corny Half-shade Polarising Appa- 

 ratus, provided with wedge compensation. 



Franz Schmidt and Haensch, Berlin. 



This apparatus differs from the foregoing in having the double plate re- 

 placed by a double Nicol's prism. In using it both fields of the apparatus 

 are adjusted to equal half darkness, instead of equal colour, as in the " Soleil." 

 The double Nicol prism was first proposed by Professor Yelett, of Dublin, 

 and employed by Professor Corny in Duboscq's polariscope for circular 

 polarisation, known as saccharometre & penoinbre. The improvement in the 

 instrument exhibited by Messrs. Schmidt and Haensch consists in combining 

 with it their wedge-compensation, so as to obtain the advantage of lineal 

 readings. The instrument recommends itself for dark solutions ; it is indis- 

 pensable for colour-blind operators, and it prevents the colour- weariness to 

 which the healthy eye is liable. Its sensitiveness perceptibly exceeds that of 

 a Soleil. 



876. A Wild's Polari-Strobometer. 



Franz Schmidt and Haensch, Eerlin. 



877. Jellett-Corny Polarising Apparatus, constructed 

 for circular polarisation. Franz Schmidt and Haensch, Berlin. 



878. Mica-preparations of mono- and bi-axial mica, for 

 polariscopes. (See Mineralogy.) Max. Raphael, Breslau. 



879. Mica-preparations of foliaceous mosses ("Laub- 

 moosen"), Algal, &c., for microscopes. Max. Raphael, Breslau. 



879a. Quartz Axis Plates. M. Lutz, Paris. 



879b. Amethyst cut parallel to the axis. M, Lutz, Paris. 



880. Dichroscopic Lens. 



Four Nicol's prisms. 



One Grazmowski prism. 



Two Tourmlines parallel to the axis. 



Fluor-spar of M. Bertrand's arrangement. 



Quartz and mica for compensating the refraction of crystals. 



Heated crystals, felspar, gypsum, carbonate of lead. 



One blue glass, red glass, and violet glass. 



Laurent. 



882. Table Polariscope, made by the contributor when a 

 youth. Rev. Nicholas Brady, M.A. 



The interest of this instrument consists in showing with what simple 

 materials a student can construct n fairly useful apparatus, the divided circles 

 being common stamped protractors; the clamping screws, teapot thumb- 

 screws, and the mountings of the lenses ordinary simple microscope frames. 



