70 On the F»rm of Spectacle- Glasses. 



If a person places a meniscus spectacle-glass before his 

 eye of a long focus, and views, towards its cxtrcmitv, one 

 or more lighted candles, he will observe the flames tinged 

 with prismatic colours, but not so with the usual convex 

 class. In this position both the effects of longitudinal and 

 lateral aberration are produced. 



Two double convex glasses placed together in one cell 

 contain less aberration than one glass of the same diameter 

 and focus ; and two plano-convex glasses, with their con- 

 vex sides placed together in one cell, give still less aberra- 

 tion. It is loss of light only that can be objected to. They 

 arc too weighty to be adopted in spectacles, but in the eye- 

 pieces of large telescopes for viewing celestial objects they 

 have been used to great advantage. To the engravers, mi- 

 niature-painters, and other artists, they are most useful, as, 

 by short foci, and large apertures, they give them the most 

 distinct view of a large surface placed before them. 



For the satisfaction of any intelligent person who may be 

 disposed to have an ocular proof of the properties of glasses, 

 as herein advanced, I have constructed a frame containing a 

 double convex, a plano-convex, a meniscus, and two plano- 

 convexes with their convex sides to each other, all of the same 

 diameter ; and by which may be seen that the greatest peri- 

 pheretical indistinctness is with the meniscus glass.- — This 

 apparatus will be shown by application at our manufactory 

 in Holborn. 



The meniscus, as a figure for a spectacle-glass, I consider 

 very objectionable. To afford a large field of view its dia- 

 meter must be considerable, which, for a short focus, will 

 increase thickness, protuberance outward, and weight, and, 

 in concave glasses, occasion the frames to be made thicker. 

 The glasses will be more liable to be scratched and broken 

 than those of the common form, and, when the frames are 

 metallic, more liable to increase than diminish tliat inde- 

 lible mark made on the nose by the weight of the frame, so 

 frequently complained of by persons who wear spectacles 

 constantly. A great deal of superfluous light also passing 

 through the glasses must be evidently prejudicial ; and it ap- 

 pears to me that the concave figure of the inner side of the 

 Uieniscus will act as a powerful reflector to condense the 

 ravs of light and heat upon the eyes, and ultimately prove 

 thereby of serious injun,\ 



I have in my possession a meniscus spectacle-glass, taken 

 from a spectacle frame, which I can prove to have been 

 made a great many years a^^o : and finally, as it is neither 



new 



