374 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Note on some Meteorological Uses of the Polariscope.*— L. Bell, 

 as the result of certain observations made at Mount Moosilauke, New 

 Hampshire, was led to think that the polariscope might have some use 

 in meteorological prognostics. Atmospheric haze is well known to be due 

 to suspended particles of one sort or another, and haze which produces 

 polarisation as well as the ordinary sky polarisation, is due to particles 

 small compared with the wave-length of light. The polariscope 

 integrates the effects of such particles along the line of sight. The 

 process of increasing nucleation, which results in cloud formation and 

 frequently in subsequent rain, was found to be accompanied by a 

 fall in polarisation, and its progress could be well followed by the 

 polariscope. 



Reichert's Novelties in Mirror Condensers.* — 0. Heimstadt 

 describes several new forms of mirror condensers which have been 

 recently brought out by the firm of C. Reichert, of Vienna. 



Mirror Condenser with variable disk-diaphragm.— This is shown in 

 fig. 94, the principle being that of the well known iris diaphragm but 



Fig. 94. 



Fig. 95. 



with reversed action. The small plates P of the disk B are projected 

 over the rim of the top plate so soon as the lever H is rotated in the 

 required direction about the axis T. The effect of the lever action is to 

 extend outwards the little plates of the disk so that their rims approxi- 

 mately form a circle which can attain to the size of the opening of the 

 mirror condenser. The above apparatus is listed by the maker as 

 " Mirror Condenser C," and is optically the same as the mirror con- 

 denser A. With dry objectives the lever H is rotated rightwise, and 

 with immersion systems leftwise. It is to be noted that this appa- 

 ratus does not secure an absolutely dark field, because the aperture 

 of the condenser cannot be greater than the aperture of the objective ; 

 some light other than that diffracted by the ultra-microscopic particles 

 will therefore reach the eye. But this fact does not constitute a dis- 

 advantage, for it is found that the image is brighter and the higher 

 powers of the immersion system have more effect. 



* Proc. American Acad, of Arts and Sci., xliii. (1908) pp. 407-12 (1 fig.). 



* Zeitschr. wiss. Mikrosk., xxiv. (1907) pp. 233-42 (7 figs.). 



