224 SEC. 7. LIGHT. 



889a. Ladd's Polariscope, consisting of a bundle of glass, 

 selenite design, and Nicol's prism, &c. fjT. Ladd fy Co. 



2531. Photograph of a Wild's Polari-strobometer, for 



determining the rotation at different temperatures. The tube 

 containing the liquid is surrounded by a jacket, through which 

 water of a given temperature flows. The apparatus is manu- 

 factured by Messrs. Hermann and Pfister, Bonn. 



Prof. H. Landolt, Aix-la-Chapelle. 



2532. Photographs of a simple Polari-strobometer with 

 two Nicol's prisms, constructed for holding tubes, one meter in 

 length, containing the liquids which can be placed in a water-bath ; 

 and a blow-pipe lamp, over which is suspended a platinum gauze 

 cage for holding the sodium salt which is used for producing the 

 monochromatic sodium flame. 



Prof. H. Landolt, Aix-la-Chapelle. 



The lamp is manufactured by Dr. Meyerstein of Gottingen, and by Feld- 

 hausen, philosophical instrument maker, Aix-la-Chapelle. 



2533. Photograph of the same apparatus, provided with a 

 short tube for holding the liquid. A bottle containing a solution 

 of potassium dichroraate is interposed between the tube and the 

 sodium flame, to ensure a purer monochromatic light. 



Prof. H. Landolt, Aix-la-Chapelle. 



III. PHOTOMETERS. 



891. Great Atmospheric Photometer. De la Rive model, 

 designed by M. Thury, and constructed by the Geneva Associa- 

 tion for the Construction of Scientific Instruments. 



De la Rive Collection. The property of Messrs. Soret, 

 Perrotj and Sarasin, Geneva. 



This apparatus is particularly intended to measure the transparency of the 

 atmosphere. It is used for simultaneous observation, with one eye, through 

 two similar eyepieces of two similar objects placed at different distances. 

 The difference of brightness and of tint between the two reflections indicates 

 the effect of the intervening stratum of air. The computation of this differ- 

 ence is arrived at by equalising the two images by means of diaphragms of 

 different aperture, and of glass plates variously tinted. The instrument is 

 composed of a telescope with a single eyepiece, and two objective tubes, 

 of which the angular distance can be varied between and 29. A system 

 of four total reflection prisms unites the two divergent cones in the eyepiece. 

 The apparatus is movable round three different axes, and may be worked in 

 the most varied directions. Graduated circles measure these different angular 

 motions. It is a general photometer, and can be specially used as an astro- 

 nomical photometer. De la Hive has effected with it a long series of obser- 

 vations on the transparency of the air. (See Comptes Rendus, vol. 63, 

 p. 1221.) 



