162 PRACTICAL TREATISE ON THE MICROSCOPE. 



right in the lower lens, and to the left in the upper, and, as 

 the most refrangible of the coloured rays the blue, is altered 

 in its covirse at each bending, and falls near the margin of the 

 second lens, where the refraction is more powerful than in the 

 centre, the blue and red rays will emerge very nearly parallel, 

 and, consequently, colourless to the eye ; thus the chromatic 

 aberration is ahnost, if not entirely destroyed, whilst the 

 spherical has been considerably diminished by the circum- 

 stance that the side of the pencil which passes one lens 

 nearest the axis, passes the other nearest the margin. But 

 however carefully a doublet of this form may be constructed, 

 there must, of necessity, be some small amount of error ; the 

 central pencil will occupy the same relative position in both 

 lenses, the correction of this, consequently, will be imperfect, 

 and all those rays intermediate between the centre and the 

 margin will vary according to their distance from one or the 

 other ; but allowing this, the doublet is, nevertheless, vastly 

 superior to any single lens of the same power, and may be 

 made to transmit a pencil of an angle from 35° to 50°, with- 

 out any very sensible errors, and to exhibit most of the usual 

 test objects. 



Another most important improvement in the magnifying 

 power of the simple microscope is the triplet of Mr. Holland, 

 before described in pages 31 and 65 ; in this instrument three 

 lenses are employed, the two lowest being placed close to- 

 gether, and the stop between them and the third. The first 

 bending of the rays being accomplished by two lenses instead 

 of one, the aberrations are so much diminished, that the 

 second bending neutralizes them entirely. The triplet, 

 though composed of three lenses, is, nevertheless, a doublet in 

 its action, and is capable of transmitting a pencil of sixty-five 

 degrees with distinctness and correctness of definition; with 

 it we arrive at the highest stage of perfection that the single 

 microscope has ever yet attained, as the errors of spherical 

 and chromatic aberration, to which all single lenses are more 

 or less subject, are nearly destroyed ; but however well either 

 the triplet or the doublet may perform separately, they will 

 not answer well as object-glasses to the compound microscope. 



