214 HOW TO SEE WITH THE MICROSCOPE . 



For example : By knowing the thickness of the cover, 

 one is enabled to approximately adjust the objective at 

 sight, and thus save time. We have thousands of 

 mounted objects in our cabinets, and .every cover has 

 been measured with all the accuracy obtainable. Those 

 who have long had their attention called to this item, 

 can, by dint of practice thus obtained, tell closely the 

 thickness of the cover by simply feeling it, and this, let 

 me assure the novice, is an accomplishment worth hav- 

 ing. Said the veteran microscopist of New York, Rev. 

 Dr. Armstrong, to the author, not long since, " It's 

 astonishing how fast you work. You seem to be in 

 perfect harmony with the objective." Now the secret 

 of the fast work noticed by the doctor lay in the fact 

 that I knew the thickness of all my covers, and was 

 thus enabled to place the collar of the object-glass very 

 closely in position at the very start; and not only this; 

 I was, for the same reason, posted as to the use of water 

 or glycerine. Now, to take a case from practice: Sup- 

 pose I desired to examine a brand new mount. Let it 

 be a difficult diatom this time. First, I run my finger 

 over the cover and instantly discover that it is a thin 

 one, say about like those used on the Moller plates. 

 Now, if I elect to use the one-sixth objective, I know that 

 this cover is too thin for Avater immersion ; hence glycer- 

 ine is chosen. I know, too, that over such a cover, and 

 with the glycerine intermedium, the objective will " cor- 

 rect" some three or four divisions from closed; there- 

 fore the collar is at once placed near such position. 

 Now, on looking through the tube at the object in posi- 



