Wells.] 374 [April 8, 



Section of Microscopy. April 8, 1874. 



Mr. Bicknell in the chair. Eleven persons present. 



Mr. Stodder exhibited scales of Petrobius maritimus and 

 Amathusia Horsfeldii, to show that the so-called " beads " 

 were the results of imperfect observation and illumination. 



Mr. Bicknell exhibited and explained his achromatic con- 

 denser, made by Mr. Tolles after the design of Mr. Bicknell. 

 Its focal distance is fe, and its aperture 150°. Its most im- 

 portant variation from other condensers is in the position of 

 the stops, the diaphragm-plate being placed close to the 

 front lens, which gives a power of controlling the illuminat- 

 ing ray greatly superior to that possessed by other condenser. 



Mr. Samuel Wells exhibited a heliostat, remarkable chiefly 

 for the small expense at which it was constructed. 



It was made from a marine clock, capable of running like a watch, 

 in any position; the hands being removed, a pulley of -| in. diameter 

 is slipped on to the arbor of the hour hand ; on the wood work at the 

 top of the clock is fastened bearings for a small shaft, carrying at its 

 upper end the plane mirror intended to follow the movement of the 

 sun. On this shaft is a pulley one inch in diameter, deriving motion 

 from the pulley on the hour hand arbor by a cord. A support at- 

 tached to the side of the clock carries a subsidiary mirror directly 

 above the revolving mirror. The clock is hung on a board, hinged 

 so as to be capable of elevation to an angle equal to the complement 

 of the latitude. The face of the clock is turned to the north. The 

 revolving mirror is adjusted to the declination of the sun so as to re- 

 flect the day to the north. The ray is received on the subsidiary 

 mirror, which reflects it in any required direction. 



The cost of the heliostat was less than twenty dollars, and its per- 

 formance sufficiently accurate for microscopic purposes. 



