120 ARRANGEMENT OF APPARATUS, ETC. 



sleeves were only mentioned on account of being 

 easily made, and their completely excluding actinic 

 light. 



Whichever method is adopted, before centering 

 with the low power objective, we may substitute the 

 dry front of the i, if it possesses one, for the top 

 combination and pin-hole cap of the French con- 

 denser. It can be easily made to fit it ; if too large, 

 by a cardboard ring slipping on the screw of the 

 French lens ; or if too small by placing it in a 

 cardboard tube fitting round the outside of the two 

 French lenses. 



This arrangement may appear subversive of the 

 rules already laid down with regard to the achroma- 

 tism of the condenser, as the front is supposed to 

 consist of a single uncorrected piano convex crown. 

 Practically this objection does not hold good, as the 

 light is monochromatic. Spherical aberration is 

 produced but the errors from it are counter-balanced 

 by the increase of NA. which now bears almost ex- 

 actly the proper proportion to that of the objective 

 itself. It will be seen that the definition is perfect, 

 and the ease with which the sunlight illumination is 

 obtained is remarkable. The low power objective 

 being attached to the microscope, we proceed in the 

 same manner with the centering as in the last 

 method. 



The object is placed on the stage of the micro- 

 scope: it should be remembered that when using high 

 power such as the one-eighth imm. it is better to 

 select a flat thin object, as the angular aperture aud 



