Photographing Histological Preparations hy Sunlight. 171 



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 fi'om two 

 to five hundred diameters I use a ^th of an inch objective, without 

 an eye-piece, 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 cor- 

 rections of which I find are generally well suited to photographic 

 requirements. 



Now, with a |th objective and the arrangement above described, 

 the field is so brilliantly illuminated that the eye cannot safely be 

 permitted to look down the tube. The image is therefore received 

 on 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 the ordinary solar microscope. With these arrangements, 

 the cardboard placed from two to four feet fi'om the stage of the 

 microscope is sufficiently well illuminated to permit distinct vision, 

 even when objectives of the shortest focus are used and powers of 

 five to ten thousand diameters obtained. While the object is thus 

 seen on the white screen in its natural colours, the cover corrections, 

 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 achromatic condenser, and di'aw 

 down the velvet hood which prevents leakage of Hght from about 

 the microscope into the dark room ; then going to the plate-holder 

 I make the final focussing in the usual way 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 ex- 

 perienced some difficulty in giving the right exposure ; for as the 

 time required was but a fraction of a second it was 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 exposures the 

 hehostat might be dispensed with, and I found on trial without it 

 that a large right-angled prism used in the position of total re- 

 flexion, or even an ordinary mirror, gave excellent results ; the 

 exposures being even shorter than when the hehostat was used, 

 since there was but a single reflexion. I could not satisfy myself, 

 however, that the quality of the pictures difiered from those 



