484 Mistd^ahH&itk 



are a variety of simple means by which the magnifying powers of 

 microscopes may be ascertained and compared, it is to be regretted 

 that science has not yet suggested any easy and infalUble mode of 

 testing their powers of definition and penetration. Definition appears 

 mainly to depend upon the amount of exactitude with which the 

 spherical and chromatic aberrations of various parts of the instrument 

 are balanced and corrected. Penetration chiefly depends upon the 

 "angle of aperture" of the object-glass, i. e. upon the angular width 

 of the luminous pencils passing from each point of the object through 

 the lens. The brightness of the image increases as the square of the 

 diameter of the lens or mirror which produces it ; hence it is obvious 

 that of two lenses of equal focal length and refracting power, that 

 which is the wider and can transmit the broader pencil of rays, will 

 give the brighter image. If the effective diameters of two such lenses 

 be = 1 and 3, there will be nine times as much light in the image 

 formed by the second as in the image formed by the first. Of course 

 there will be, in the image formed by the second lens, parts clearly 

 visible, which in the image formed by the first lens were too feebly 

 illuminated to affect the retina of the observer. When the aberra- 

 tions are perfectly corrected, i. e. when the rays passing from the 

 object through the centre and margins of the lens are refracted 

 without chromatic dispersion and to the same foci, the maximum of 

 defining power is attained, and the penetrating power of the lens is 

 likewise a maximum for that angle of aperture. It does not, however, 

 follow that a lens of greater angular aperture is not superior in 

 penetration, nor that a lens of exquisite penetrating power, i. e. 

 capable of exhibiting the most feebly illuminated parts of an image, 

 must necessarily be one of good defining power, i. e. capable of ex- 

 hibiting sharp outlines on bodies of extreme minuteness, or of re- 

 solving or separating the dots, stripes, etc., of the more difficult test- 

 objects. With these preliminary observations, which may be re- 

 garded as an imperfect abridgement of different chapters in Harting's 

 excellent treatise, we proceed to lay before our readers his very 

 ingenious and beautiful method for testing the qualities of definition 

 and penetrating power. — Editor. 



I conclude by noticing another method of testing the optical power 

 of the instrument, which, although rather troublesome, appears to 

 me among the best, permitting us, as it does, to ascertain with a 

 great degree of accuracy and certainty, the utmost limits of pene- 

 trating and separating power possessed by a microscope, and hence 

 easily to express numerically its optical qualities in the most varied 

 circumstances. 



This method consists simply in subjecting to observation under the 

 microscope the dioptric images of certain miuute objects instead of the 

 objects themselves. These images can be diminished at pleasure by 

 withdrawing to a distance from the lens the object which forms them ; 

 and hence we have it in our power to measure the extreme limits at 

 which the object continues to be visible. 



For the formation of the dioptric images, achromatic object-glasses 



