VISUAL MAGNITUDE. 



about 2000, and that of the sun about 887000 miles, the visual 

 diameter of the one and the other measures about half a degree. 



The magnitudes of objects, as they appear with magnifying 

 glasses, are all visual and not real. When an object seen by the 

 interposition of such an instrument is said to be magnified so many 

 times, it is therefore meant that it is so many times greater than 

 it would be if the same object were seen with the naked eye ; but 

 since it has been shown in our Tract on the eye, that the visual 

 magnitude of the same object seen with the naked eye varies, being 

 greater as its distance from the eye is less, it follows, that the visual 

 magnitude seen with the naked eye is an indefinite and variable 

 standard, and in order that the visual magnitude of an object 

 taken as the standard of magnifying power should be definite, it 

 is necessary that the distance at which the object is supposed to be 

 viewed by the naked eye should be stated. "When, however, a 

 person without any previous scientific instruction views an object 

 with a magnifier, he becomes instantly conscious of its amplifica- 

 tion ; that is, that it appears larger than it would appear if he had 

 viewed it without the interposition of the magnifier. The question 

 is then, at what distance from his eye such a person would sup- 

 pose the object to be looked at without the magnifier ; and the 

 reply which has been generally given to this question is, that he 

 would suppose it to be viewed at that distance at which he would 

 see it most distinctly. 



6. This being admitted then, microscopists have generally agreed 

 that the visual magnitude viewed with the naked eye, which should 

 be taken as the standard of comparison in expressing the effect of 

 magnifiers, is that which the object would have when viewed at the 

 distance at which objects are most distinctly seen. 



7. But here another difficulty arises. In the first place, the dis- 

 tance at which one individual can see an object most distinctly is 

 not the same as that at which another will see it most distinctly ; 

 thus, while a far-sighted person will see most distinctly at the 

 distance of 15 or 16 inches, and cannot see at all at the distance 

 of 5 or 6 inches, a near-sighted person will see most distinctly at 

 the latter distance, and only confusedly and indistinctly at the 

 former. But even the same individual will see the same object 

 most distinctly at one distance when it is strongly illuminated, 

 and at a much less distance when it is feebly illuminated. 



The distance of most distinct vision is therefore a variable and 

 uncertain standard of comparison. 



8. But there is one thing which is perfectly definite and Qer- 

 tain. The visual magnitude of an object, at a given distance, 

 is always the same, and quite independent of the powers and qua- 

 lities .of the eye which views it ; it may, or may not, be distinctly 



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