II. SELECTORS. 221 



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 Apparatus, 



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 Jelett, of Dublin, 

 and employed by Professor Corny in Duboscq's polarise ope for circular 

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

 instrument exhibited by Messrs. Schmidt and Haeusch 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 prevents the colour- weariness to 

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

 a Soleil. 



876. Wild's Polari-Strobometer. 



Franz Schmidt and Hac?isch 1 Berlin. 



877. Jellett-Corny Polarising Apparatus, constructed 

 for circular polarisation. Franz Schmidt and Hacnsch, Berlin. 



877a. Handy Folariscope with Kicol prisms to show rings 

 in bi -axial crystals. W. Previte Orton. 



This instrument was designed by the exhibitor and made for him by 

 Pastorelli. Its object is to utilize the Nicols of a small microscope so as 

 to show the effect of polarised light in a biaxial crystal ; and further, to do 

 this in a handy manageable and inexpensive form. 



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. 



b79b. Amethyst cut parallel to the axis. M. Lutz, Paris. 



880. Dichroscopic Lens. 



Four Nicol's prisms. 



Prazmowski prism. 



Two Tourmalines parallel to the axis. 



Iceland 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 green glass. 



Billet lens on stand. 



Laurent. 



881. Ellipsometer. Before the eyepiece of the glass a 

 double refracting prism is made to turn until a wire, moving per- 



