452 SUMMARY OF CUERENT RESEARCHES RELATING TO 



plate placed in the frame of the plate-holder after drawing out the two 

 shutters and putting a 50 grm. weight on the opposite side to prevent the 

 camera from tilting over. The figure represents the apparatus duriug 

 the adjustment. ° 



A simple method for determining the correct time of exposure was 

 communicated to the author by Dr. Knoevenagel, of Linden near 

 Hanover j it consists in partially drawing out the under shutter after 

 certain intervals of time, whereby the object to be photographed is 

 brought upon the same plate under three or four different times of 

 exposure. As an explanatory example the author takes the case of a 

 very dark green preparation, using a dry objective which for a clear 

 colourless preparation required an exposure of an hour's duration • the 

 under shutter, before the beginning of the illumination, is drawn out a 

 third, again a third after the expiration of an hour, and quite drawn out 

 alter a further half-hour, after which the exposure is continued for 

 another half-hour; thus, on the plate there will be parts of the picture 

 under one, one and a half, and two hours' exposure. 



The power of accommodation of the human eye is a trouble to the 

 maker and observer of photomicrographs. The eye sees, in fact, several 

 planes, of which the plate only fixes one; thence arises the practice of 

 the microscopist of rapid up-and-down focusing, by which an impression 

 ot relief is given to the object. In the appearance of a photograph there 

 IS thus something lacking which gives rise to a feeling of discontent 

 until one has learnt to look at it in the right way. It is a matter of 

 general experience that the drawing of a microscopic object often leads 

 to a correct observation of it ; for, whether consciously or unconsciously, 

 it IS possible to draw together in one plane images seen in different 

 planes. If it is desired to demonstrate any one detail of a certain small 

 part of a preparation, it is therefore desirable to make a drawing as well 

 as a photograph. 



Bezu, Hausser, and Co.'s Photomicrog^raphie Apparatus.*— MM 

 Bezu, Hausser et Cie. have just brought out a photomicrographic 

 apparatus (fig. 69), of which the following is a description :_ 



It IS constructed on the vertical system, and is composed of three 

 parts. The first is a strong stand, made of oak plank, 55 cm. long and 

 45 cm. broad, supported on four cast-iron feet about 20 cm. high. In 

 the middle of this oak stage is placed the Microscope upon a copper 

 stand with four legs, and this is moved up and down by means of a screw 

 placed between the legs. The Microscope is firmly fixed to the copper 

 stand by jamming the horse-shoe between grooves and holdino- it in 

 position by a screw behind. 



To the oak stand is fixed the table carrying the camera. This table 

 IS made of oak planking 35 cm. long and 25 cm. broad, and its legs of 

 cast iron are 45 cm. high. At its centre is a circular aperture for a copper 

 tube lined with black velvet. In the latter the Microscope tube works. 



The camera is composed of three parts: a cubical box, the sides of 

 which are 12 cm., a bellows, and a frame for the opaque glass screen. 

 1 fie latter is kept m position by two iron supports, which are really 

 continuations of the hind-legs of the table, and moved up and down by 

 a rackwork arrangement. When the bellows is fully extended the 

 diameter of the image is 18 cm. 



* Journ. de Mierograpliie, xiii. (1889) pp. 189-91 (1 fig/. 



