328 SCIENTIFIC APPARATUS. 



obtain a sharp image free from false colour. In using simple 

 lenses both llie spherical and chromatic aberration:; may be 

 reduced by limiting the aperture with a stop, or by using only the 

 central part of the lens ; but though we thus gain in definition, 

 we lose in brilliancy. 



In order to overcome these difficulties, various combinations of 

 lenses with spherical curves have been adopted, which more or less 

 completely overcome spherical aberration. Such doublet or triplet 

 lenses mounted on a suitable stand have all the advantage of 

 great portability, and since the object is seen in its natural posi- 

 tion and not inverted as with a compound microscope, it is much 

 more easy to manipulate or dissect, than when every movement 

 must be made in the reverse direction to that which appears 

 natural. On the contrary, with anything like high powers, the 

 object approaches most inconveniently close to the lenses, and 

 it is trying to the eye to look through the necessarily very 

 small opening, so that, except for particular purposes, there is 

 no doubt that the compound instrument is by far the best form of 

 microscope. 



COMPOUND MICROSCOPES. 



If a single or double convex lens be held at its focal distance 

 from a lighted candle, an inverted image of the flame may be 

 thrown on a distant wall. Having thus formed such an image, we 

 might magnify it further by looking at it through another lens. 

 But it is not necessary that the image should thus be formed on 

 any material surface. It exists as it were in space, and may be 

 viewed by means of a lens placed beyond the image, in the line 

 of the beam of light. Two simple lenses so arranged, one acting 

 as object glass and the other as eye-piece, would form a rudimen- 

 tary compound microscope, but with such an arrangement the 

 spherical and chromatic aberrations would be so great that nothing 

 at all like good definition could be obtained. The earliest forms 

 of compound microscopes were constructed on this principle, or 



