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V. A Defcription of a new Conf ruction of Eye-glaffes for fuch 
Pel e [copes as may be applied to Mathematical Inflruments, 
By Mr. Ramfdcn ; communicated by Sir Jofeph Banks, Bart . 
P. R. S. 
Read December 19, 1782, 
rpO correct the errors in eye-glaffes,arifing from their fpheri- 
A cal figure, and alfo from the different refrangibility of 
light, it has been held abfolutely neceflary to have two, placed 
in luch manner, that the image formed by the objeCt-glafs of 
the telefcope fhould be between them ; but in thofe telefcopes 
that are applied -to mathematical inflruments, the interference 
of the firft eye-glafs before the image is formed is productive of 
many bad confequences ; fhould that eye-glafs have the lead: 
fhake or motion whatever, it totally alters the adjuflment of 
the mftrument ; and the diminifhing alfo of the image by this 
pofition, obliging us to fhorten the focus of the nearer eye- 
glafs, the wires in the focus of the telefcope are thereby conn- * 
derably more magnified than they would have been with the 
fame power, had both the eye-glafl'es been put between the 
image and the eye. 
Many defeCts in the micrometer with moveable wires are 
• caufed by the conftruCtion of the eye glaffes of the telefcope to 
which it is applied. If only one eye-glafs is ufed, the field is . 
fo contracted^ that.it is impofifible to meafure the diameter of the 
fun or moon with preciiion, if the telefcope magnifies above 30 
times ; 
