Chemistry and Physics. 499 



aperture 8 cm ) forms a real image of this screen in front of the 

 observing telescope in such a manner as to cover one-half of the 

 full aperture (3*5 0m ) of the objective. The distance between the 

 lens and telescope is about 5 meters, and the magnifying power 

 of the latter is X 24. Not far from the emergence surface of the 

 first lens a specially designed cell is mounted on the prism of a 

 Pul f rich refractometer. The cell contains the specimen of glass 

 to be studied and a certain solution {vide infra). A second 

 opaque screen with a vertical straight edge is mounted in front 

 of the observing telescope so that its plane coincides with the 

 plane of the image of the first screen. The planes of both 

 screens are at right angles to the common optic axis of the cen- 

 tered system of lenses. The second screen is provided with a 

 rack and pinion combination which enables the observer to impart 

 a slow horizontal motion to the edge and so cause it to gradually 

 eclipse the telescope objective. 



The immersion fluid finally selected was an aqueous solution of 

 mercury potassium iodide, commonly called Thoulet's solution. 

 The index of refraction for D light of this liquid can be decreased 

 continuously from 1*72 to 1*33 by increasing the proportion of 

 water. The solution possesses two advantageous properties : 

 (a) the excess of water can be driven off by heating, consequently 

 the double salt can be used repeatedly, and (b) it does not attack 

 the Canada balsam employed in cementing together the walls of 

 the cell. On the other hand, the mercury compound exercises 

 marked absorption at the more refrangible end of the visible 

 spectrum. The curve given in the paper shows that the percent- 

 age transmissions for the four lines, C, D, F, and G', are approxi- 

 mately 70, 63, 16, and 2, respectively. Although the temperature 

 coefficient of refraction of Thoulet's solution is very high 

 (— 0-0006 per degree C.) the author states that no difficulty is 

 experienced on this account. 



The experimental procedure consists essentially in moving the 

 screen next the observing telescope slowly across the beam of 

 light and noting whether the field of view darkens uniformly. 

 When the solid specimen in the cell has a refractive index differ- 

 ent from that of the surrounding liquid the portion of the field of 

 view corresponding to the image of the object under investiga- 

 tion will not darken simultaneously with the rest of the field. By 

 varying the concentration of the liquid a match or balance may 

 be quickly obtained and then the index of the liquid (and hence 

 of the specimen) is determined at once with the refractometer. 

 The accuracy of which this method seems to be susceptible is 

 about two units in the fifth decimal place. For example, the 

 index for D light for a certain glass prism, having a refracting 

 angle of 10°, was found to be 1*51492 by the new method and 

 1*51490 by direct use of the refractometer. — Phil. Mag., xxxii, 

 p. 409, Oct., 1916. H. s.u. 



7. Fluorescent Vapors and their Magneto-optic Properties. — 

 The theoretical aspect of the beautiful experiments by R. W. 



