MAGNIFYING POWER. 



discuss. It will suffice to say that there is such a limit. If the 

 visual angle formed by lines diverging from the eye to the extre- 

 mities of the object be within this limit, the object will not be per- 

 ceived ; or, to speak with more rigour, its magnitude and form 

 will not be perceived.* 



In such cases, therefore, the visual magnitude of an object, 

 without the intervention of the microscope, must be understood 

 to mean the angular divergence of the rays which would be drawn 

 from a point placed at ten inches from the object to its extremities. 

 This would be the visual magnitude of the object " if it could be 

 seen " at that distance. 



In fine, therefore, the definition of the magnifying power of a 

 microscope will be clear, distinct, and adequate, if it be stated 

 thus : It is the quotient which would be obtained by dividing 

 the visual magnitude of the object, as seen in the microscope, by 

 the visual magnitude which the object would have to a naked eye 

 placed at ten inches distance from it, supposing the eye to have 

 sufficient sensibility to perceive it at that distance. 



Every one is more or less familiar with real magnitude, so that 

 when an object of ordinary dimensions is placed before them they 

 can give at least a rough estimate of its actual dimensions. The 

 same facility of estimating visual magnitude does not exist, 

 although, in fact, we receive the impressions of visual much more 

 frequently than those of real magnitude. The estimate of visual 

 magnitude, however, enters into all microscopic inquiries as an 

 element and condition of such importance, that all those who use 

 the instrument, whether for the purposes of serious research or 

 rational amusement and instruction, would do well to familiarise 

 themselves with it. Some observations illustrative of such sen- 

 sible impressions will therefore, we presume, be not unacceptable 

 to our readers. 



66. Our great familiarity with real magnitude arises from our 

 intimate knowledge of certain standard units by which it is 

 counted. There is no one, however little educated, that has not 

 a pretty clear notion of the length expressed by an inch, a foot, 

 and a yard. Let us see whether we may not enable any one with 

 common attention to acquire an equally clear notion of the 

 standard units of visual magnitude. 



Every one is familiar with the apparent magnitude of the disc 

 of the full moon. It is visible to the whole world, and seen for 

 several nights in each month during the entire life of every indi- 

 vidual. Now it happens that the visual magnitude of its diameter 



* The fixed stars are visible as mere luminous points, but their forms 

 and magnitudes are not perceivable, owing to the extreme smallness of 

 their visual angle produced by their enormous distances. 



61 



