ARTIFICIAL LIGHT. 197 



could get no light through the objective, showing that 

 in this case the accessory was a reflex and no mistake; 

 or, suppose that our best efforts were only rewarded by 

 a dim view of the diatom seen doubled up endwise amid 

 a plexus of indeterminate undefinable diffraction lines, 

 the entire field miserably illuminated, of a muggy, 

 smoky, dingy yellow, as if a piece of yellow flannel 

 had been used in place of the lamp, and that we could 

 only get this much in just one solitary position of the 

 lamp ; then the experiment is quite as interesting, quite 

 as valuable, and if made in due time will amply repay 

 its cost. 



As a matter of course, the lower the balsam aperture 

 the lower will be the grade of its work with the " re- 

 flex ;" nevertheless, the point of maximum performance 

 can be ascertained by the method above given, and in 

 the testing of object-glasses I make it a point to look 

 after the balsam angle and the point of maximum aper- 

 ture at the same sitting, the only additional trouble 

 involved being the change in the thickness of cover, or, 

 as in the instance named, substituting glycerine in place 

 of water. 



We have entered into these details, feeling assured 

 that the facts are worth knowing, and that many there 

 be who have not given these things due attention. Of 

 those who have visited us, eight out of ten saw the work- 

 ing of the "reflex" lor the first time, while without 

 exception all have seemed greatly pleased and interested 

 with such comparative experiments as we have just de- 

 scribed. Let the reader rest a moment here and I will 

 relate a little incident : 



