480 THE AMERICAN NATURALIST [Vol. LV 



the experiment need never be repeated again. The table itself 

 applies onl}- to an Orthonon plate used without ray-filter. For 

 use with Cramer's rayfilters the factors given in this table must 

 be multiplied by the factors shown in my first paper. 



The so-called " Pointolite " is an excellent source of light for 

 low powers. It requires about 5-10 times longer exposures. In 

 our laboratory we use the Complete Illuminating Apparatus with 

 D.C. open arc* After some cxperimeiitino' it was found that the 

 most satisfactory arrangpnirrit of this ilhiminating system is 

 when the arc is moved forward to within l^i/o inches from the 

 first condensing lens, a water i-ell placed between second and 

 third condensing lenses, in contact with the mounting of both, 

 and the distance between the third and fourth condensing lens 

 fixed at 15 inches, while the distance from the latter to the 

 microscope table is 1^2 inches. Under these conditions the ex- 

 posure is double that obtained with the microprojector for 

 which the table in this paper is given. I omit giving a separate 

 table for this system, because any one may obtain it in a moment 

 by simply doubling the values given here. But it must be re- 

 membered that change in the arrangement of the illuminating 

 system, no matter how small the deviation, will result in wrong 

 values. 



Alexander Petruxkevitch. 



