BECK'S VERTICAL ILLUMINATOR. 221 



ror being at the same time brought into play, the illu- 

 mination may be almost instantly changed to that 

 of transmitted light, or from thence back to reflected 

 light again. This cross-questioning under two methods 

 of illumination is often of great advantage. 



Per contra : The vertical illuminator has but one 

 drawback, and that is rather a serious one, to wit: Ob- 

 jects to be displayed under it must be mounted dry, and 

 also contact the cover. Hence it will be seen that a 

 large portion of histological, pathological, as well as 

 other permanent mountings, are excluded from use. 

 Many of these can be temporarily prepared for study, 

 and it is hoped that the attention of observers will be 

 enlisted in this direction. Admitting the serious char- 

 acter ot the objection named, let us bear in mind that 

 there remain countless fields of research wherein the 

 "Beck" will certainly prove an instrument of the 

 greatest value. 



The tyro will find the Beck illuminator a difficult in- 

 strument to use, and his first attempts will probably 

 result in failure. Nor is it an easy task to give any 

 instruction in writing that will be of much aid. The 

 instrument not being a costly one, and likely to be gen- 

 erally used when it shall become better known, we will 

 try and furnish a few hints that perchance may prove 

 of value to the beginner. 



The novice will do well, at his first attempts, to select 

 a dry mount, one that he is perfectly familiar with, and 

 preferably scales from the lepidoptera; a dry mount of 

 podura will answer very well, indeed. Select, also, the 



