208 J. C. Draper—Preparation of Cylinders of 
Art. XXIX.—On the preparation of Oylinders of Zirconia for the 
Oxy-hydrogen Light; by JOHN CHRISTOPHER DraPER, M.D., 
LL.D., Professor of Natural History in the College of the 
City of New York. 
position in the optical axis of the apparatus. 
the experimenter has a good heliostat, the light from the 
sun fulfills these conditions better than any artificial light; but, 
experience teaches the unwelcome lesson that though sunlight 
is preferable to any other, it scarcely ever happens that it is 
available at the time it is wanted. The weather is almost cer- 
tain to be either cloudy or hazy just at the hour when it is 
desired to make an important demonstration, and the leeturer 
is obliged to postpone it, thereby lessening its value, and often 
entirely losing its effect. The selection of the best artificial 
light therefore becomes a matter of importance to those who 
desire to secure the advantages to be derived from the success- 
ful demonstration of such microscopic objects as reparations 
of animal and vegetable tissues, animalcules, the circulation of 
the blood, ete. 
