24 MICROSCOPIC OBJECTS. 



equal penetration, that which has the lowest magnifying 

 power is the best. 



If the penetrating power of a microscope could be as 

 easily determined as that of a telescope, there would be 

 no difficulty in the matter ; a simple measurement of the 

 diameter would be sufficient. But such is not the case ; 

 the penetration in the microscope is as the angle-of- 

 aperture. Thus, suppose the aperture be 10 degrees; if 

 the diameter be kept the same and the focal length of 

 the lenses reduced to one half, the angular aperture 

 will be nearly doubled. 



A full account of angular aperture, with a list of its 

 amount obtained by lenses of different foci, will be found 

 in the Third Edition of the * Microscopic Illustrations.' 



It is proper to state, that about the period of the 

 fabrication of the first achromatic object-glass for a 

 microscope, by Dr. Goring, scientific men in all parts of 

 Europe were engaged in improvements on that instru- 

 ment ; but all their talents and energies were directed to 

 other parts of the microscope. High magnifying power 

 and a large flat field of view appear to have been their 

 chief aim. In Scotland, the Royal Society of Edinburgh 

 had a microscope constructed between twenty and thirty 

 feet in length ; but did not apply achromatic object- 

 glasses : it follows, therefore, that the Council did not 

 deem them of consequence. In Germany, the experi- 

 menters approached nearly the right direction ; for they 

 strung five or six object-glasses together ; but their 



