THE MICROSCOPE. 13 



It exhibits, therefore, many of the usual test-objects in a very 

 beautiful manner. 



The next step in the improvement of the simple microscope 

 bears more analogy to the eye-piece. This improvement was 

 made by Mr. Holland, and it consists (as shown in Fig. 6) in 

 substituting two lenses for the first in the doublet, and retain- 

 ing the stop between them and the third. The first bending, 

 being thus effected by two lenses instead of 

 one, is accompanied by smaller aberrations, 

 which are therefore more completely balanced 

 or corrected at the second bending, in the 

 Fig. 6. opposite direction, by the third lens. This 



combination, though called a triplet is essen- 

 tially a doublet, in which the anterior lens is divided into two. 

 For it must be recollected that the first pair of lenses merely 

 accomplishes what might have been done, though with less 

 precision, by one; but the two lenses of the doublet are opposed 

 to each other; the second diminishing the magnifying power 

 of the first. The first pair of lenses in the triplet concur in 

 producing a certain amount of magnifying power, which is 

 diminished in quantity and corrected as to aberration at the 

 third lens by the change in relation to the position of the axis 

 which takes place in the pencil between what is virtually the 

 first and second lens. In this combination the errors are still 

 further reduced by the close approximation to the object which 

 causes the refractions to take place near the axis. Thus the 

 transmission of a still larger angular pencil, namely 65, is ren- 

 dered compatible with distinctness, and a more intense image 

 is presented to the eye. 



Every increase in the number of lenses is attended with one 

 drawback, from the circumstance that & certain portion of 

 light is lost by reflection and absorption each time that the ray 

 enters a new medium. This loss bears no sensible proportion 

 to the gain arising from the increased aperture, which, being 

 as the square of the diameter, multiplies rapidly; or, if we esti- 

 mate by the angle of the admitted pencil, which is more easily 

 ascertained, the intensity will be as the square of twice the tan- 

 gent of half the angle. To explain this, let D B (Fig. 7) repre- 

 sent the diameter of the lens, or of that part of it which is 



