THE MICROSCOPE. 31 



each other ; yet it is, nevertheless, found to be essential to a 

 good doublet. 



A, C, Fig. 7, represent the lenses of the doublet, and B is 

 the diaphragm or stop. The manner in which the light is 

 refracted by this instrument, is shown by the lines proceeding 

 from each end of the object, 0. The dotted lines represent 

 the blue or most refrangible rays of the spectrum ; the others 

 are the red rays. Those rays which pass through the centre 

 of the lens, A, are caused to pass through the hole in the dia- 

 phragm over to the margin of B, and those nearest the margin 

 of A, pass next the centre of B ; and so become nearly cor- 

 rected : the imperfection of one being made to counteract that 

 of the other. 



An improvement was made upon this by Mr. Holland, and 

 is called Holland's Triplet. It consists of a doublet in place 

 of the first lens, A, in the last figure ; retaining the stop be- 

 tween it and the lens C. This form is the highest stage of 

 perfection which the simple microscope has ever yet attained. 

 The great objection to its use, however, is, that it must be 

 brought into such close proximity to the object, that it is im- 

 possible to cover such object except with the thinnest mica, 

 which is objectionable on account of its liability to be scratched. 



Before dismissing the subject of single microscopes, it may 

 be well to remark, that for a low magnifying power, a double 

 convex lens is the best to use ; but for medium or high powers, 

 a plano-convex lens, with the convex side towards the object ; 

 or one of the doublets just described; is preferable. 



THE COMPOUND MICROSCOPE 



Consists essentially of two convex lenses ; an object-glass and 

 an eye-glass ; as represented in Fig. 8. 



