34 PICTURING MIRACLES 



soon wore it out so that if a light burned out, a belt 

 broke, or the motor ran hot or any of a dozen other 

 things happened or if the plant refused to grow, 

 the picture was spoilt and it all had to be done over 

 again. Then as it was necessary to change the speed 

 intervals, that meant changing the exposure which 

 was very difficult to control to get a uniform nega- 

 tive of many hundreds of pictures. The exposure is 

 made by an adjustable slot moving by the film. If 

 the intervals were short, it moved rapidly and at 

 twenty or thirty minute intervals you could hardly 

 see it move. So control of the exposure was done by 

 narrowing the width of the slot a very difficult way 

 of getting the correct amount of light on the film. If 

 the slot were i/ s " wide and it took several minutes 

 to revolve by the film, it was quite a different matter 

 from one which was 2" wide passing in perhaps less 

 than a second. 



So the engineering problem was: How to get uni- 

 form exposures regardless of the intervals at which 

 they were taken, and to be able to change the ex- 

 posure intervals at will, depending on how fast or 

 how slowly the plant or subject was growing or 

 changing its position. I bought a small telechrome 

 motor for $4. Its shaft projected out about an inch 

 and made a revolution in one minute when con- 

 nected up with the electric light current. A small 



