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On a New Micrometer. 

 By George J. Burch. 



(Communicated by Mr. T. Curties ; Bead March 22nd, 1878.) 



Plate IV. 



It is not every Microscopist who can afford a thoroughly efficient 

 Micrometer, and it is not always that the possessor of one can find 

 time to use it. 



A really good working Micrometer should bear the same relation 

 to the microscope that the two-foot rule does to the carpenter's 

 bench — it should be both cheap and handy. 



The spider-line Micrometer costs quite half as much as a good 

 many people lay out on their microscopes, and the cheaper eye-piece 

 scales used in its stead are at best only adjusted for one particular 

 power, while for all others they are about as handy as the car- 

 penter's rule would be if its measurements had to be reduced to 

 inches by a table, or by Rule of Three. The result is that nobody 

 uses a Micrometer if he can help it, unless he has nothing else to do. 

 What is wanted is a Micrometer that shall 

 1. — Cost but little ; 



2. — Measure accurately with any power ; 

 3. — Read in decimals of an inch or millimetre, without any 



calculation ; 

 4. — Apply equally well with all kinds of illumination ; 

 5. — And take no more time to put on than the mere changing 

 of an eye-piece. 

 To invent such a Micrometer is a problem which I have had before 

 me ever since the summer of 1872, and I now submit to you an 

 instrument which bids fair to solve it. 



Noticing one day some scratches on the lamp glass, focussed by 

 the achromatic condenser, it struck me that I might use the image 

 of a scale on glass formed in the same way as a Micrometer. 



I saw that I could easily adjust the size of this image to coincide 



e 



