194 THE AMERICAN MONTHLY [June 



are of value and for the practical and more general ap- 

 plication of pliotomicrography, while the results must be 

 of the best, the time consumed must be small, the man- 

 ipulations must be simple, and the apparatus must be 

 one with which photographs can be taken at any time. 



In the early days of photography, when the wet plate 

 only was available, sunlight was necessary to photo- 

 graphic processes, and the traditions derived from its 

 use cause many still to consider it essential to the pro- 

 duction of high-class photomicrographs. With the in- 

 troduction of the dry plate, artificial light became avail- 

 able, and in spite of its small actinic power, relative to 

 that of the sun, certain advantages connected with its use 

 have given it many advocates. It is not necessary to 

 enter into an extensive comparison of tlie relative optical, 

 visual, and actinic value of sun and artificial light. Much 

 has been written in favor of one and derogatory to the 

 other. The fact remains that equally good work has been 

 done with both. But for practical work artificial light 

 has many advantages. Sunlight is uncertain; it varies in 

 intensity from hour to hour of the day and with the time 

 of year. It is apt to be obscured for days together or by 

 passing clouds at critical moments, and, at most, is avail- 

 able but for a few hours of the twenty-four. Also, the 

 sun is constantly changing its position relative to the in- 

 strument, and when used for all except the highest power 

 of the microscope, its image when focused on the plane of 

 the object covers too small a field, and the heat and un- 

 desired colored rays have to be filtered out with light 

 and heat filters. It is true that the latter disadvantages 

 can be overcome by suitable but complicated apparatus, 

 but the great objection of unavailability, except at un- 

 certain times, still remains, and in consequence when 

 sunlight is depended on, many valuable records are lost 

 from inability to photograph objects at once after their 

 observation. For these reasons sunlight is not available 



