THE COMPOUND MICROSCOPE 25 



on which they are made. But I counsel the pur- 

 chaser to consider only such objectives as are made 

 by firms of standing reputation, and in making his 

 choice to obtain, if possible, the advice of a practical 

 microscopist. Objectives of ancient date, offered 

 second-hand, even if made by reputable makers, 

 should be carefully tested. Indeed, any but 

 recently made glasses should be regarded with 

 suspicion. 



Achromatism, akeady spoken of, may be taken 

 for granted in all good class objectives. Other 

 points to be considered are definition, whereby a 

 sharp line in an object is rendered in a sharp image 

 by the objective ; the power of penetration, giving 

 depth of focus ; flatness of field ; and working dis- 

 tance between objective and object. Of course, the 

 higher the power of the object glass, the less is the 

 working distance. Roughly speaking, objectives 

 are rated according to their working distances ; thus 

 a J^inch glass is more powerful than a 1-inch glass, 

 and the distance between the under side of the lens 

 and the object is greater, when focussing has been 

 done, in the case of the l-inch than in that of the 

 J-inch. But because an objective is classed as a 

 J-inch or 1-inch, it does not follow that its working 

 distance is exactly J inch or 1 inch ; it wiU probably 

 be considerably less. 



As to what " powers " the beginner needs, this 

 depends upon the kind of work he proposes to do. 

 If he is simply going to amuse himself and his friends 

 in a desultory fashion, a ^-inch objective might 

 serve him as well as any other — perhaps better, 

 because he will find it easy to focus and illuminate. 



4 



