MAGNIFYING POWEK. 



THE MAGNIFYING POWEK. 



63. It has been well said, that a question clearly put is half 

 resolved. There is no term in microscopic nomenclature so- 

 familiar to the ear, and so flippant on the tongue, as the " magni- 

 fying power;" yet there is none respecting which there prevail 

 so much confusion and obscurity. The chief cause of this is the 

 neglect of a clear and distinct definition of the term. 



It has been already shown, that the magnitudes observed with 

 the microscope are visual, not real. We can say that such or 

 such an object seen in the microscope has a magnitude of so many 

 degrees, but not at all one of so many inches. Strictly speaking, 

 the same is true of all objects seen in the ordinary way ; but in 

 that case the mind is habituated to form an estimate of their real 

 magnitudes, by combining the consideration of their apparent 

 magnitudes with their distances. It is true that we are uncon- 

 scious of the mental operation from which such estimates result, 

 but it is not the less real. Our unconsciousness of it arises from 

 the force of habit, and the great quickness of the acts of the 

 mind. Every one who has been familiar with intellectual pheno- 

 mena knows that such unconsciousness is found to attend all such 

 acts as are thus habitual and rapid. 



64. But when objects are looked at in a microscope, the mind 

 not only does not possess the necessary data to form such an 

 estimate, but the conditions under which the visual perceptions 

 are formed are so unusual, and, so to speak, unnatural, that it is 

 incapacitated to form an approximate estimate even of the visual, 

 to say nothing of the real, magnitude of the object of its 

 perception. 



The visual magnitude of an object, as seen in a microscope, is 

 the angle of divergence of lines supposed to be drawn from the 

 eye to the limits of the imaginary image formed by the eye-glass, 

 which is the immediate object of perception. When we say, 

 therefore, that the instrument has such or such a magnifying 

 power, every one will comprehend that it is meant that this visual 

 magnitude is so many times greater than the visual magnitude 

 which the object would have, if it were seen in the usual way 

 without the interposition of any optical expedient. 



So far all is clear, and so far there can be no difference of 

 opinion on the point, provided only that the latter member of the 

 sentence be clearly defined. What is the " visual magnitude 

 seen in the usual way ?" There are many ways of looking at an 

 object, and "the usual way" depends much on the magnitude 

 of the object. We can see well enough the dome of St. Paul's 

 Cathedral at the distance of half a mile, while we cannot see a 



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