140 C. F. ROUSSELET ON THE ROUSSELET COMPRESSORIUM. 



under observation with a low power, the animal in swimming 

 away soon turns round, when it is immediately again secured by 

 a turn of the screw. 



In 1900 Mr. G. H. J. Rogers introduced a modification of the 

 above compressor, in which the cover-glass was held in position 

 by two india-rubber bands instead of being cemented, claiming 

 that it was thus easier to replace a broken cover-glass. I pointed 

 out at the time that this very slight advantage was much more 

 than counterbalanced by the introduction of two serious defects : 

 in the first place, the cover-glass was no longer sufficiently firm 

 and rigid for delicate manipulation ; and secondly, the com- 

 pressorium became quite unsuitable for use with water and oil 

 immersion lenses, as these fluids could run under the brass ring 

 and eventuallv mix with the water on the slide. The removal 

 of a broken cemented cover is so readily effected by slightly 

 heating it over a lamp, that no modification is required on that 

 account — none at least that cancels two very essential qualities 

 of my model. 



In spite of these obvious defects, this modified compressor 

 found entrance into several opticians' catalogues under the 

 title, " Improved Rousselet Compressor," the india-rubber bands 

 having meanwhile been replaced by two screws. These screws 

 have introduced a fresh defect, because the arm has to be raised 

 considerably before it can be turned aside. There is absolutely 

 no reason why any one wanting such a compressor should not 

 have it ; but I think I must ask these opticians to be good 

 enough to call their modified model by some other name, for 

 I have always disapproved of it for the reasons stated above, 

 and it is not my model and very much less an "improved" 

 form of it. 



. Quehett Microscopical dab, Ser. 2, Vol. IX., No. 50, April 1905. 



