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would open a large field of enquiry;) but to try to assist 

 mechanical contrivance in its fabrication. To this end, I 

 tliink it fit to add a remark, on the property of the pencils 

 of rays, respecting the latitude of each of them, Avhere 

 they fall on the pupil of the eye; first discovered by the 

 oreat Mr. Iluygens: as I suspect, that the inconvenience 

 he mentions, as resulting from a certain bi'eadth of the 

 pencil, may casually exceed the limit stated by him, and 

 may admit of a practical remedy. 



He observes, that if the latitude or breadth of the pen- 

 cils, at the pupil of the eye, be very small, (so as not 

 to exceed, if I remember right, the sixtieth part of an 

 inch,) the vision, by the telescope or microscope, will be 

 indistinct; so that, unless -the pencils be of greater breadth 

 than this space, at the place of the eye, the instrument 

 will be defective: and he attributes this to something in 

 the natural conformation, or in the humours of the eye; 

 which, consequently, will admit of no remedy. On this 

 may I presume -to obser\ e, timt -the latitude of the pencil, 

 as it enters the eye, is the same as that with which it 

 falls on the last <eye-glass ; and, tliat the effect of it, which 

 Mr. Huygens attributes to the eye, may, therefore, as na- 

 turally, be. attributed to. the eye-glass, as to the eye; espe- 

 cially when an anatomical dissection of it will demonstrate, 

 that the perfect transparency of its humours, and exqui- 

 site polish of its cornea, or outer coat, (not to talk of 

 its .achromatic property,) far exceed, in this optic instru- 

 ment oi' the Deity, any thing that can be manufactured 



by 



