157 



Under these circumstances, it is highly desirable that the conden- 

 ser should be constructed specially with reference to the aperture of 

 the object-glass with which it is intended to operate ; and for a rea- 

 son to be given immediately, it will be seen that cutting off some of 

 the rays, in order to make a condenser work with objectives of very 

 much larger aperture, although quite practicable, and even generally 

 in use with the parabolic condenser, is not nearly so advantageous 

 as the use of a separate condenser for every object-glass. This ob- 

 servation applies equally to the parabolic as to the sphsero- annular 

 condenser. The absolute quantity of light capable of being conden- 

 sed on any object, is limited by the diameter of the condenser ; and 

 the area of circles being proportionate directly as the squares of their 

 diameters, it follows that any reduction of the effective reflecting por- 

 tion near the circumference, diminishes the quantity of light in a much 

 greater degree than would be the case from a corresponding reduc- 

 tion of the reflecting portion nearer the centre : but if a condenser 

 be constructed for a small aperture, in order to make it act with an 

 object-glass of large aperture, it is the circumferential rays which 

 must be cut off, and the amount of light is thus materially reduced, 

 with a power requiring on the contrary more. Again, if a condenser 

 be constructed for a large aperture, and it be used with an objective 

 of much smaller aperture, there is no means of reducing the obliquity 

 of the light, and consequently of satisfying the condition that the 

 aperture of the condenser should only be a few degrees differing from 

 that of the objective ; and this I find practically a much greater in- 

 convenience than the defect previously adverted to. 



As the cost of the glass part of the condensers is very moderate, I 

 therefore strongly advocate the use of a separate condenser for every 

 object-glass, of high power at least. 



I have now to explain the mode of designing the sphaero- annular 

 condenser for any given aperture, which is most readily accomplished, 

 and with sufficient accuracy for all practical purposes, by means of 

 enlarged diagrams. 



Having drawn a perpendicular line, d b (fig. 1. page 158), make the 

 angles x d b, y db, each equal to half the given aperture ; and draw 

 the line x y at right angles to d b, so that it is equal to the intended 

 diameter of the condenser. Draw y o parallel to d b, and bisect the 

 angle d y o by the line c y. Draw a line through the point d, and 

 another, n, at such distance below as is fully equal to the thickness 

 of a glass slider, or something more, and let both be parallel to x y ; 

 then with the centre d, and some distance, d m, beyond the lines n 



