56 THE MICROSCOPE. Apr., 



Focusing Upward. 



By R. H. ward, M. D., 



TROY, N. Y, 



It is often advised to focus downward towards the slide, 

 using great care not to go too far, until the object is 

 actually seen. More prudent advice would be to set the 

 tube too low for its focal distance, in every case, while 

 observing it from the side by looking through, horizon- 

 tally towards the light, between the objective and the 

 slide, and then find the object by focusing upward while 

 looking down the tube. After this, the whole thickness 

 of the object can safely be examined by focusing slowly, 

 and in many cases almost constantly, through it, forward 

 and backward. 



Some experts of great experience, prudence and self- 

 control, persons of microscopic touch, and to whom cau- 

 tion and accuracy have become instinctive, may find the 

 focus in almost any way, scarcely knowing or caring how. 

 One of the commonest of these expedients is, while work- 

 ing the rock with one hand, to feel the way with a finger 

 of the other hand, touching lightly the side of the objec- 

 tive at its lower edge, while the same finger or the next 

 one, according to circumstances, projects downward to 

 touch lightly the top of the slide. This gives a positive, 

 and for low and medium powers, a sufficiently accurate 

 knowledge of the varying distance from the objective to 

 to the slide. But some of those heedless persons who, 

 with the best of intentions, are always doing something 

 wrong, until their suffering friends are strongly tempted 

 to wish they were enemies instead, will either push the 

 slide off from the stage to be broken by the fall, or else 

 misjudge the indications of their touch until they hear 

 the glass cracking under the pressure of the objective, 

 when they will exclaim, more truly than they mean, "Oh ! 

 I didn't think ." 



To nearly all beginners, and to an uncertain proportion 



