198 OPTICAL PROJECTION 



and when found will be very little altered for the same 

 objective, unless the thickness of the slides should differ very 

 greatly. But the adjustment should be tested for possible 

 improvement from time to time, and will require slight 

 change with every change of power. This however is not all ; 

 the position of the lime itself should now be adjusted more 

 precisely, and too much care cannot be devoted to these points. 

 The position of the lime is tested by sliding the tray of the 

 jet slightly in and out. It will usually be found that a slight 

 alteration improves the brightness of the object materially. 

 The lime thus readjusted on an object, after the substage 

 condenser has been adjusted, will be in its position for average 

 work. But whenever the result seems not satisfactory with 

 any slide, it is always worth while to try afresh whether the 

 lime is in its best position ; and occasional adjustments of this 

 and of the substage condenser will be well repaid. 



There is one general rule in these occasional adjustments 

 of the light, without which no projection microscope can do 

 its best with varying powers, though so long as the same 

 power is used all through, they are unnecessary. The lime 

 will have to be pushed up nearer the lantern condensers, for 

 lower powers, and drawn rather farther back with higher 

 powers. The distance is not great ; but the difference in the 

 light on the screen produced by these readjustments is won- 

 derful. Now and then, after the lime has been readjusted, a 

 very slight readjustment of the substage condenser also will 

 still further improve the effect. 



When the microscope is once started and precisely adjusted, 

 no alteration will be needed whilst the same power is adhered 

 to ; but whenever the power is changed, and especially 

 whenever the substage condenser has to be changed, re- 

 adjustment of the light should be tried for as described above ; 

 also when, without changing the objective itself, an amplifier 

 is either put on or taken away, the substage condenser will 

 require re-focussing upon the object. The lime will require 



