328 Improvement in the Form of Spectacle Glasses w 



best for spectacles, as the double concave of like figure js 

 the best to help short-sighted persons." 



But although it may be very true that such a form of 

 glass was best calculated for the object-glass of a telescope, 

 previous to the celebrated discovery of the achromatic ob- 

 ject-glass by the late Mr. Dollond ; yet, whatever advan- 

 tages might at any time be expected from the telescopic 

 object-glass so shaped, these were not to be obtained by a 

 similar construction in spectacles, as may easily be seen by 

 considering the difi'erent uses of the respective instru- 

 ments. 



In a telescope, in the first place, our view is necessarily 

 confined to a very small distance on each side of the axis ; 

 and secondly, every part of the object-glass contributes to 

 the distinctness of any ob}ect viewed. 



It is under these circumstances alone that the proportion 

 of the curvatures above mentioned might be proper for a 

 single object-glass, as being capable of collecting into the 

 same focus the rays that fall on every part of it parallel to 

 the axis. 



By spectacles, on the contrary, objects are to be viewed 

 if possible in every direction in which they might be seen 

 by the naked eye, which is often far removed from the 

 centres of the glasses ; consequently, a construction that is 

 calculated to represent correctly central objects alone can- 

 not be the most advantageous. 



In these also the portion of the glass employed at once is 

 scarcely larger than the pupil of the eye ; so that any en- 

 deavour to procure the concurrence oi all parts of a glass 

 in any one effect is evidently superfluous, and may also be 

 shown to be prejudicial. 



It is therefore proposed to remedy the imperfections ob- 

 servable in the spectacle glasses hitherto generally used 

 upon a principle susgestcd bv this latter consideration, 

 which presents an opportunity, by a different eonstruction, 

 of rendering objects in all directions distinct. 



The alteration requisite for this purpose is extremely sim- 

 ple, and easily intelligible. Supposing an eye to be placed 

 in the centre of any hollow globe of glass, it is plain that 

 objects would then be seen perpcndiaaarly through its sur- 

 face in every direction. Consequently, the more nearly 

 any spectacle glass can be made to surround the eye, in the 

 manner of a globular surface, the more nearly will every 

 part of it be at right angles to the line of sight, the more 

 uniform will be the power of its different parts, and the 

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