304 THE AMERICAN MONTHLY [Oct 



tempting to do this with high powers, for fear as before 

 explained, of reducing the N. A. of the objective, which 

 of course means spoiling its definition ; should the light 

 still be too intense with low powers, however, then smoked 

 glasses or monochromatic ones should be interposed be- 

 tween the illuminant and the condenser. 



Lastly it is important that the condenser should be suit- 

 ed to the objective so far as relates to its N. A. Theo- 

 retically, both should have the same aperture, but practi- 

 cally, no objective will stand this except perhaps when 

 looking at or photographing bacilli : but on the other hand 

 it is necessary in order to obtain good results not to use 

 the condenser with too low a N. A. 



As before stated, a small amount of cutting down by 

 the iris is justifiable under certain conditions, and most 

 authorities set the maximum limit at about a third of the 

 diameter of the back lens of the objective, excepting 

 when employing the microscope on bacilli. Then lastly, 

 to obtain the best results, certainly when photographing 

 with the microscope, an achromatic condenser is a neces- 

 sity, and the use of an apochromatic one is better still. 

 Messrs. Powell & Lealand have recently brought out an 

 excellent dry apochromatic condenser of -95 N. A., whose 

 performance leaves little to be desired. It may seem as 

 if these remarks on the condenser were uncalled for con- 

 sidering the nature of the subject under consideration; 

 but attention is called to all these details as it is positive- 

 ly certain no objective will perform at its best, unless the 

 greatest care be exercised in the application and suitabil- 

 ity of the condenser and the arrangement of the light. 



Before actually comparing lenses, suitable test objects 

 should be chosen and carefully studied. The Proboscis 

 of the blow-fly for low powers is good, but besides that 

 a Podura Scale and some finely marked diatoms must be 

 obtained; and lastly, if it can be possibly procured, a 

 really good specimen of the Amphipleura pellucida mount- 



