Note on the Proper Definition, &c. By Prof. E. Ahhe. 351 



This misleading idea, resulting from the common expression, is 

 eliminated by defining the 10 in. merely as the distance from the 

 eye at which the image is measured — whether it he a distinct or an 

 indistinct image. For if an observer, owing to the accommoda- 

 tion of his eye, obtains a distinct image at a distance of 10 feet, I 

 may nevertheless assume a plane at a distance of 1 in. from the 

 eye on which the distant image is virtually projected, and measure 

 the diameter of that projection. Now this diameter is strictly the 

 same as the diameter of that image, which another observer 

 would really obtain with distinct vision at that same distance of 

 10 in. The only difference is, that in the former case we must 

 take the centres of the circles of indistinctness instead of the sharp 

 image-points in the latter case. 



If the conventional length oil = 10 in. is interpreted in this 

 way (as distance of projection, independently of distinct vision) the 

 absurdity at least of a real influence of the accommodation on the 

 power of a Microscope is avoided. It becomes obvious, that for 

 long-sighted and for short-sighted eyes the same N must indicate the 

 same visual angle of the enlarged objects, or the same magnitude of 

 the retinal image, because it indicates the same diameter of the 

 projection at 10 in. distance. (See fig. 48.) 



