MICllU-PHOTOGKAPHY BY DIRECT SUNLIGHT. 73 



tions, focussing, management of the achromatic condenser, and 

 selection of the portion of the preparation to be photographed 

 are readily managed. When all is satisfactory, I insert an 

 ammonio-sulphate cell between the large lens and the achro- 

 matic condenser, and draw down the velvet hood which 

 prevents leakage of. light from about the microscope into the 

 dark room ; then, going to the plate-holder, I make the final 

 focussing in the usual Avay on the ground glass, or on plate 

 glass with the help of a focussing glass, according to the 

 nature of the object. 



" With powers of five hundred diameters, or less, I at first 

 experienced some difficulty in giving the right exposure ; for 

 as the time required was but a fraction of a second, it is a 

 matter of some difficulty to regulate it with precision. At 

 length I succeeded by arranging a sliding shutter, with a 

 transverse slit of variable width, so adjusted as to fall with 

 its own weight before the tube of the microscope, the exposure 

 being made during the passage, and the time of exposure 

 regulated by the width given to the slit. 



" Of course it occurred to me that for such short exposure 

 the heliostat might be dispensed with, and I found on trial 

 without it that a large right-angled prism used in the position 

 of total reflection, or even an ordinary mirror, gave excellent 

 results ; the exposures being even shorter than when the 

 heliostat was used, since there was but a single reflection. I 

 could not satisfy myself, however, that the quality of the 

 pictures differed from those obtained Avith the help of the 

 heliostat, except, perhaps, that in ceftain cases the prism 

 seemed to offer advantages which will be referred to here- 

 after. Under these circumstances the heliostat appears 

 desirable for ordinary use, since the solar pencil being thrown 

 in a constant direction, the trouble of adjusting the illumina- 

 tion of a series of objects is considerably diminished ; but I 

 have convinced myself that equally good pictures, even with 

 very high powers, may be produced without itj a circum- 

 stance of considerable interest where motives of economy 

 preclude the microscopist from procuring this convenient 

 instrument. 



" The special device of placing the large condensing lens 

 so as to bring the solar rays to a focus before ])assing through 

 the achromatic condenser had been suggested many years 

 before by the late Rev. J. B. Reade, as an improvement to 

 the solar microscope, Avith the special object of preventing the 

 concentration of heat rays and any consequent injury to the 

 object." — (' Silliman's American Journal,' October, 1871.) 



Development of Algae. — Professor A. M. Edwards has com- 



