3Qfi "THE AMERICAN MONTHLY [Oct 



sidered synonymous, although some writers do draw a 

 ditference between them. It will be seen that the flat- 

 ness is ofttimes of larger area — reaching further to the 

 edges in the achromatic than with the apochromatic, but 

 that the definition in the centre is superior in the apochro- 

 matic to that in the achromatic image. In some "inches" 

 by first-class makers, such as in the new inch by Ross .3 

 N. A., and one by Leitz & Reichert, it is not easy with- 

 out close attention to discern any inferiority of image to 

 that presented by the apochromatic, for the improvement 

 afforded by that type of lens does not become so very 

 evident when compared usually with excellent achromat- 

 ics using low eyepieces, until a little higher power is em- 

 ployed, although apparent in photography. But note one 

 thing, that the curvature of the field and the consequent 

 loss of definition at the edges presents this difference in 

 the two cases. In the achromatic no amount of refocuss- 

 ing will render the periphery sharp, but with the apo- 

 chromatic the slightest turn of the screw renders the 

 edges of the field as good as the centre — hence the mind 

 can rapidly acquire a mental picture of the whole image 

 by a touch of the screw. With the achromatic, however, 

 if the edges be required to be examined, that portion of 

 the specimen must be brought to the centre of the field 

 before focussing renders the definition as good as possible. 



2. Blackness of the Shadows or Dark Portions op 

 THE Image. — -Notice whether the dark ribs, or tracheae as 

 they are called, of the proboscis are really black, and see 

 which lens renders them the more so. Look carefully to 

 see if there be a haze over the whole image like a veil 

 which no management or adjustment of the light, con- 

 denser, or otherwise will get rid of except by sacrificing 

 the definition of the objective by closing the iris dia- 

 phragm. The apochromatic to the practised eye will here 

 show a superiority. 



3. Brilliancy of Image. — Here a good deal of prac- 



