OBJECTS BY USE OF GRADUATED PINE ADJUSTMENT. 283 



The following observations confirm the results obtained above. 



The depth of a cell on a slip was first measured with the cell 

 empty, and without a cover glass. Cover glasses of various thick- 

 nesses were then successively placed over the cell, and the depth 

 of the cell again measured in each case. 



Similar measurements were then made with the cell filled with 

 water and with glycerine. 



The results obtained are tabulated below : 



The measurements of the depth of a cell just given, although 

 quite sufficient to confirm the theoretical deductions made above, 

 are not quite the kind of measurements which would usually have 

 to be made in practical work. We should more likely be con- 

 cerned with the thickness of some portion of an object which was 

 quite immersed in the mounting medium. The following is an 

 example of such a measurement. 



After some search for a suitable object, the claws on the hind 

 legs of a cat flea, CtenocepJialus felis, mounted in pure glycerine, 

 were chosen. In this particular mount the legs happened to be so 

 arranged that one claw on one foot was lying horizontally, while 

 one claw on the other foot was directed vertically. Since the two 

 claws on the same foot are approximately of equal length, and since 



