104 



SUMMARY OF CTKliENT RESEARCHES RELATING TO 



large one. by turning the other wheel and rod. A Zeiss planar lens of 

 short focus is used for moderate enlargements, and a Microscope objective 

 for enlargements on a greater scale. As the pitch of the screw threads 

 which are cat on the long rods is only one-fifth inch, the focusing can be 

 accomplished as easily and accurately as with a rack-and-pinion. The 

 object is illuminated by a Nernst lamp of 120 candle-power, provided 

 with condensing lenses. The part of the object which it is desired to 

 photograph is brought into the field by means of a total reflection prism 

 and a Microscope eye-piece attached to the camera, as the great length 

 of the apparatus makes it impossible to move the object by hand while 

 observing its image on the ground-glass screen. In addition to these 



Fig. 15. 



two forms of apparatus, the photographic laboratory of the School of 

 Mines contains an excellent equipment, including two dark-rooms pro- 

 vided with all necessary apparatus, and a third and larger dark-room 

 containing an enlarging camera and a paper-holder for the production of 

 enlarged positives, 80 by 40 cm. (12 by 16 in.) in size. The camera and 

 the paper-holder are mounted on a single frame, which is supported by 

 springs on four light masonry columns. In consequence of this arrange- 

 ment the tremors produced by the passage of railway trains and trolley- 

 cars affect the negative, the lens, and the paper simultaneously and to the 

 same degree, and therefore do not diminish the sharpness of the repro- 

 duction. Another enlarging camera, sliding on rails, and provided with 

 a lens of 12 in. focus, is used for the reproduction of drawings, measuring 

 12 by 16 in. The same apparatus, with the addition of two arc-lamps 

 and a series of screens, is employed for instructing the future engineer 

 in the methods of photo-engraving. 



