236 OPTICAL PROJECTION 



is accomplished by focussing a plate of glass with a few lines 

 marked upon it, and adjusting the light for this ; and then 

 making every smoked plate occupy the same position as nearly 

 as possible, throughout the series of demonstrations. 



The movements of the heart have been projected by other 

 methods. The apparatus shown in fig. 122 is extremely simple. 

 A heart freshly taken from the living frog or other animal is 

 embedded in sufficient warmed wax to form a steady base, 

 and laid on the plate a. Above it extends a light lever, fd, 



FIG. 123. Czermak's Cardioscops 



bearing an index-disc, /, at the end, or it may be a style ; this 

 has a very sensitive joint at d i, which can be adjusted length- 

 ways over the heart by the screw g, and this and the other 

 screw g also serve to adjust lever and stand on the pillar m. 

 For some experiments requiring great delicacy, it may be 

 advisable to carefully balance the arm fc, or the somewhat 

 similar arms in fig. 121, so that their weight may not inter- 

 fere with the motion. Between the heart and the lever at c 

 is adjusted a small pillar b of elder pith, with a small point at 

 each ond, which communicates the motion, the lower point 



