72 NOTES AND MEMORANDA. 



Micro-Photography by direct Sunlight. — Mr. .J. J. Wood- 

 ward, of the United States' army, has directed his attention 

 to the method of photographing histological preparations by 

 direct sunlight. To avoid the inconveniences of direct solar 

 rays it has been customary to pass them, for photographic 

 purposes, through a screen of ground glass. Mr. Woodward 

 finds that it is possible, by the use of a suitable condensing 

 lens, to dispense entirely with the ground-glass screen, and 

 thus greatly to diminish the time of exposure as well as to 

 obtain superior sharpness of definition. His method is as 

 follows : 



" The microscope being placed on a shelf at the window 

 of the dark room, and its body made horizontal, the 

 achromatic condenser is illuminated by a solar pencil reflected 

 from a heliostat upon a movable mirror outside the shutter, 

 and thence into the dark room, precisely as described in 

 my original paper upon micro-photography. No ground 

 glass is used, but instead a lens mounted in a suitable tube 

 is fixed in the opening of the shutter, through which the 

 solar pencil enters. This lens is an achromatic combination 

 about two inches in transverse diameter, and of about ten 

 inches focal length. It is placed at such a distance from the 

 achromatic condenser that the solar rays are brought to a 

 focus, and begin again to diverge before they reach the lowest 

 glass of the achromatic condenser. 



" For anatomical preparations requiring for their display 

 from two to five hundred diameters, I use an -^th of an inch 

 objective without an eyepiece, obtaining the precise power 

 desired by variations in the distance of the sensitive plate 

 from the stage of the instrument. I have lately given the 

 preference to immersion objectives, the coirections of 

 which, I find, are generally well suited to photographic 

 requirements. 



" Now with an ^th objective, and the arrangement above 

 described, the field is so brilliantly illuminated that tlie eye 

 cannot safely be permitted to look down the tube. The 

 image is therefore received upon a piece of white cardboard, 

 and, sitting by the microscope to make the adjustment, I view 

 the card with both eyes precisely as in the case of tin* 

 ordinary solar microscope. 



"AVith these arrangements, the cardboard, placed^from tAvo 

 to four feet from the stage of the microscope, is sufficiently 

 well illuminated to permit distinct vision, even when objec- 

 tives of the shortest focus are used, and powers of five to ten 

 thousand diameters obtained. While the object is thus seen 

 on the Avhite screen in its natural colours, the cover correc- 



