THE MICROSCOPE. 



distance of the object from it, the linear dimensions of the image 

 o o will be 10 times greater than those of the object ; and if the 

 focal length of the eye-glass be J an inch, the distance of most 

 distinct vision being 10 inches, the linear dimensions of o' o' will 

 be 20 times those of o o, and therefore 200 times those of the 

 object ; the linear magnifying power would in that case be 200, 

 and consequently the superficial magnifying power 40000. 



It would seem therefore, theoretically, that there would be no 

 limit to the magnifying power of such a combination ; practically, 

 however, there are circumstances which do impose a limit upon it. 

 It must be remembered that the object must always be placed at 

 a distance from the object-glass, greater than the focal length 

 of the latter, the magnifying power of the object-glass depend- 

 ing on the number of times this distance is multiplied, to make up 

 the distance of the image o o from L L ; if a very great magnifying 

 power be required, the latter distance must be a proportionally 

 great multiple of the former, and as the eye-glass must be farther 

 from the object-glass than the image, the instrument might be 

 increased to unmanageable dimensions. 



There is therefore a practical limit to the increase of the ampli- 

 fying power of the instrument by the increase of the distance of 

 the image o o from the object-glass, and consequently it can only 

 be augmented by the decrease of the focal length of the object- 

 glass, combined with a corresponding decrease of that of the eye- 

 glass. By such means, the distance of o from L L will be con- 

 tained a great number of times in o L, while the latter has not 

 objectionable length, and the distance of the eye-glass from the 

 image o o will be contained a great number of times in the 

 distance of most distinct vision. 



The eye and object glasses are usually mounted at the distance 

 of 10 or 12 inches asunder, adjustments nevertheless being pro- 

 vided, by which their mutual distance can be varied within 

 certain limits. 



6. A convex lens is generally interposed between the object- 

 glass and eye-glass, which receiving the rays diverging from the 

 former, before they form an image, has the effect of contracting 

 the dimensions of the image, and at the same time increasing its 

 brightness. The effect of such an intermediate lens will be 

 understood by reference to fig. 2, where F F is the intermediate 

 lens. If this lens FF were not interposed, the object-glass LL 

 would form an image of the object o at o o ; but this image being 

 too large to be seen at once with any eye-glass, a certain portion 

 of its central part would only be visible. The lens F F, however, 

 receiving the rays before they arrive at the image o o, gives them 

 increased convergence, and causes them to produce a smaller 

 4 



