146 The Microscope. 



a slide can readily be made by cutting a 3x1 inch slip from a 

 photo plate, exposing it to the full sunlight, developing and fix- 

 ing it, and then drawing a sharp knife across it on the film side. 

 In the transparent line thus made, Diatoms or any other objects 

 may be mounted in balsam, and covered in the usual way. Still 

 greater accuracy could be secured by the further use of a very 

 brilliant source of illumination behind the screen with a very 

 narrow vertical slit, or by the use of the tiny incandescent lamps 

 which are now available. 



WORKING DISTANCE. 



The working distance of an objective is the amount of clear 

 space between the face of the objective and the upper surface of 

 the cover glass, when the object is in focus. It is a variable 

 quantity, being affected by the special construction of the lens, 

 the length of the tube, the power and construction of the eye- 

 piece, and the thickness of the cover glass. 



The method of measurement is so simple and obvious that it 

 hardly needs explanation, but is as follows. 



I have on the body of my microscope a scale, and on the arm 

 a vernier, by means of which I can determine the position of 

 the body to the one one-thousandth of an inch. I focus the ob- 

 ject carefully and take a reading ; then with the coarse adjust- 

 ment I rack cautiously down till the front of my objective is 

 just in contact with the upper surface of the cover glass, and 

 take another reading, the difference between the two readings 

 being the working distance of the objective under those condi- 

 tions. Stands in which the fine adjustment moves the whole 

 tube, a plan introduced by the late Joseph Zentmayer in his 

 Centennial model, are especially convenient for this work, as in 

 them the fine adjustment can be used to complete the contact 

 between the objective and the cover glass, and thus decrease the 

 chances of damage to either, while in stands with the fine ad- 

 justment moving the nose-piece only, the adjustment can not be 

 used for this purpose as the movement of the objective effected 

 by it is independent of the body, and consequently would not 

 be included in the movements of the body and could not be 

 read off on the scale. 



