512 



either cannot be found or it is not in a suitable condition for use. 

 I therefore asked Mr. Ctirties to fit me a very thin plate under 

 the lengthened arm, and spring it so as to hold a cover-glass in 

 position. 



If much pressure is wanted, a thick cover-glass can be at once 

 slipped in, or, if an oil-immersion is to be used, a thin one can 

 take its place. So now I am able to get along without anxiety 

 either about scratching the front of my objective or as to where 

 a bottle of cement may be found. 



[Referring to the " improv^ed " form of compressorium for pond- 

 life devised by Mr. E. M. Nelson, and exhibited to the meeting, 

 after examination of the " improvements " I desire to observe 

 that Mr. Nelson's method of fixing the cover-glass by a thin 

 metal spring-flange extending all round the semicircular brass 

 ring, in order to save the small trouble of fixing the cover with 

 a little gold-size or other cement, has the great objection that 

 water will find its way under the flange nearly every time it 

 is used, and it being impossible to wipe it off without removing 

 the flange (by unscrewing two very small screws), the water will 

 evaporate, leaving a crust or sediment behind, which in a short 

 time will clog the flange and spoil the "improvement" ! Then, 

 whenever an oil-immersion objective is used, the oil will be certain 

 to flow under the flange and create a much greater mess, and 

 in the end creep over the under surface of the cover-glass and 

 spoil the object under examination. Further objections are 

 (1) that the cover-glass will be held less firmly than with my 

 method ; (2) that the lengthened arm carrying the cover-glass 

 will much reduce the rigidity of the arm, which already is none 

 too rigid in the shorter original form. The trouble of racking up 

 the tube to turn the double nose-piece is really very slight, and 

 is outweighed by the defects introduced. 



In view of all these objections I must continue to recommend 

 the original form of my compressor, without "improvements," 



