76 THE MICROSCOPE. 



depth of focus, that is, extreme distance of two planes, the 

 points of which are at the same time sufficiently in focus 

 for the purpose of distinct vision. This distance will 

 manifestly increase as the angle of aperture diminishes, 

 just as in a landscape camera the fore and back grounds 

 can be brought into sensible focus simultaneously only by 

 the use of a small diaphragm, which greatly diminishes 

 the angular aperture of the incident peneils. But, at the 

 same time, it must be borne in mind, that illumination, 

 cceteris paribus, increases or diminishes with angle of 

 aperture, and the best working glass will be that in which 

 a compromise is effected between these two conflicting 

 requisites. 



We entirely agree with Mr. Brooke, that "for all practical 

 purposes, except developing the markings of diatoms, an 

 objective of moderate aperture will be found most available. 

 It may reasonably be doubted whether the development of 

 the dottings of difficult diatoms is not an object rather of 

 curiosity than of utility, and whether it is worth the labour 

 that has been bestowed upon the production of glasses for 

 that especial purpose ; the labour of construction being 

 immensely augmented by the difficulty of duly balancing 

 the aberrations of the more oblique pencils. So much is 

 this the case, that in the best constructed objectives of the 



words 'penetrating power' have a definite meaning, and that the amount of 

 this power possessed by a telescope, can be obtained by calciilation. And of 

 course, this must also be true of a microscope. This power, says Mr. Ryland, 

 in a very interesting paper, must not be confused with angular aperture, which 

 has reference to the objective alone ; neither has it any connexion with either 

 definition or thickness of field. In a word, as magnifying power expresses the 

 angle subtended by an object or image at the eye of the observer, so penetrative 

 power is the measure of the angle subtended by the eye at the object, or the 

 equivalent of that angle in the case of telescopic or microscopic vision. The 

 one is the measure of size, the other of brightness. The latter, however, must 

 not be confused with ' illumination.' The one power is neither less important 

 nor less essential to distinct vision than the other. There required little mag- 

 nifying power, and there was no illumination, in the case of the church steeple, 

 still the hour could be read on the dial." 



The third power, the visual power of microscopes, is one which has rarely 

 been recognised as distinct. 



For an object to be magnified 100 times, that is &oen at 100th part of the 

 distance, it is necessary not only that the angle subtended by it at the eye the 

 magnifying power but also the angle subtended by the eye at the object the 

 penetrating power shall be increased one hundred-fold. When this is the case, 

 the visual power will be 100 also. 



If we approach an object bodily, these angles naturally increase in the same 



Eroportion, but it is not so where optical instruments are employed. R. G. 

 ylaad " On the Optical Powers of the Microscope." Micros. Jour. Science, 

 voL vii. p. 27. 



