MAGNIFYING GLASSES. 



seen, or seen at all ; but if seen, it can have but one visual magni- 

 tude. Thus an object, such as a coin, placed with its surface at 

 right angles to the line of sight, at a distance from the eye equal 

 to 10 times its own diameter, will have a visual diameter of of J , 

 and neither more nor less, no matter by what eye it is viewed. 

 Seeing, then, that the distance of most distinct vision varies with 

 different observers, and even with the same observer under diffe- 

 rent circumstances, and cannot therefore be taken as a standard 

 of reference for visual magnitude, it has been generally agreed 

 that magnifying powers shall be arithmetically expressed, by refe- 

 rence to visual magnitudes seen at 10 inches distance. Thus, if 

 we say that such or such a magnifier magnifies an object three or 

 four times, it is meant that it exhibits that object with a visual 

 magnitude three or four times as great as that which the same ob- 

 ject would have if viewed with the naked eye at 10 inches distance. 



This distance of 10 inches has not been selected arbitrarily. It 

 is considered to be about the distance at which average eyes 

 would see an object most distinctly. It has the further conve- 

 nience of lending itself with facility to calculation by reason of 

 its decimal form. In other countries, the same distance very 

 nearly has been adopted as a standard. Thus, French microsco- 

 pists take 25 centimetres, which is a very small fraction less than 

 10 inches, as their standard. 



9. This conventional standard being accepted, let us see in 

 what manner an object is made to appear magnified by the inter- 

 position of a single convex lens. 



Let E E fig. 1, represent a section of the eye, and o o' a small 

 object placed at a much less distance from the eye than is com- 



Flg. 1. 



patible with distinct vision. According to what has been ex- 

 plained in former tracts, it will appear that the cause of indis- 

 tinct vision is, in this case, that the image of o o', produced by 

 the humours of the eye, is formed not as it ought to be on the 

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