256 



USE OF PROJECTION MICROSCOPE 



[CH. IX 



389. Sharpness of the screen image. It is a mistake to think 

 that it is necessary that the screen image should be photograph- 

 ically sharp. As well said by Lewis Wright, p. 191: "A certain 

 breadth or coarseness of line is a positive advantage in the image to 

 be viewed many feet [meters] away." Of course, the image should 

 be focused as sharply as possible, but a line or structure that 

 appears perfectly distinct at a considerable distance may appear 

 indistinct when the observer is close to the screen. If the operator 

 is at a considerable distance (15 to 20 meters, 50 to 65 ft.) from the 

 screen, he will find good opera-glasses a help in getting the screen 

 images properly focused. 



FIG. 144:1. DIAGRAM TO Snow THE POSITIONS OF THE .SAME OBJECT AND 



THE SIZE OF THE SCREEN IMAGE FOR OBJECTIVES OF =JO, 2O AND IO MM. 



Focus. 



In this figure it is assumed that the object in each case is practically at the 

 principal focal distance from the objective and that, the screen distance is the 

 same for all. As the size of the image varies inversely with the distance of the 

 object from the objective it is seen that the screen must be larger for an 

 objective of short than for one of long focus in accordance with the general 

 law of the relative size of the object and image ( 392a). 



