CO? THE MICKOSCOFE. 101 



ner will often encounter difficulty. Both these points "being es- 

 sential, therefore, before discussing the magnifying powers 

 best suited to different purposes, it may be well to say a word in 

 regard to the mode in which magnifying power is always ex- 

 pressed by scientific men. 



When we look at a small object through a microscope, and 

 see it magnified to twice its length, it is evident that its breadth 

 is also magnified twice, and consequently its surface, no matter 

 what the shape may be, is magnified four times. It might also 

 be said that as we only take cognizance of bodies having a sen- 

 sible thickness, this thickness must be magnified iwice, and 

 therefore the object is magnified twice four, or eight times. The 

 latter, however, is a view which is never insisted upon, and 

 even those who claim the most for their microscopes, never do 

 more than express the magnifying power in surfaces. Scien- 

 tific men are, however, agreed that to express a magnifying 

 power in surfaces is to convey a wrong impression in regard 

 to the assistance rendered by the instrument to the natural 

 vision, for a careful study of the physiology of vision, teaches us 

 that our power to appreciate and distinguish the features of 

 any object depends upon the distances to which the characteris- 

 tic points of that object are separated, and this can be meas- 

 ured only by linear, and not by superficial units. There are 

 other considerations which lead to the same conclusion, but 

 for the beginner it is sufficient to know that all scientific mi- 

 croscopists are agreed that when the magnifying power of a 

 microscope is stated, it shall be stated in diameters, and not in 

 areas. By common consent, then, ten times means ten diame- 

 ters. And yet it is a very common thing for charlatans, and 

 those who wish to deceive the public, to say that a microscope 

 sold by them magnifies ten thousand times, or one hundred di- 

 ameters, and as " ten thousand times " is much more readily 

 appreciated by the popular mind than "one hundred diame- 

 ters," the majority of those who read such statements suppose 

 that they will be enabled to see ten thousand times more than 

 they could see with the naked eye, which assuredly is not the 

 case. In some instances these advertisers do not even state 

 the diameters. We have now before us, clipped from a journal 

 of deservedly good reputation, an advertisment which reads as 



