132 ARRANGEMENT OF APPARATUS. 



board tube fitting round the outside of the two 

 French lenses. Should the microscope be fitted with 

 a modern movable substage, and achromatic con- 

 denser with pin-hole cap, the additions mentioned 

 are of course unnecessary. 



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 plano-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 centring as in the last 

 method. 



The object is placed on the stage of the micro- 

 scope: it should be remembered that when using high 

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

 select a flat thin object, as the angular aperture ami 

 magnifying power are too great to allow of a large 

 amount of penetration. 



The part we wish to photograph being chosen, the 

 low power is removed. 



The J- objective, if a water immersion, may be 

 wetted with either water or glycerine. The former 

 is better as far as definition is concerned, but the lat- 



