350 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



in his younger years by some fatal accident or other, as is often the case in those 

 countries. 



V. A Description of a New Construction of Eye-glasses for such Telescopes as 

 mai/ be applied to Mathematical Instruments. By Mr. Ramsden. p. 94. 



To correct the errors in eye-glasses, arising from their spherical figure, and 

 also from the different refrangibility of light, it has been held absolutely neces- 

 sary to have 2, placed in such manner, that the image formed by the object-glass 

 of the telescope should be between them ; but in those telescopes that are ap- 

 plied to mathematical instruments, the interference of the first eye-glass before 

 the image is formed is productive of many bad consequences ; should that eye- 

 glass have the least shake or motion whatever, it totally alters the adjustment of 

 the instrument ; and the diminishing also of the image by this position, obliging 

 us to shorten the focus of the nearer eye-glass, the wires in the focus of the 

 telescope are thereby considerably more magnified than they would have been 

 with the same power, had both the eye-glasses been put between the image and 

 the eye. 



Many defects in the micrometer with moveable wires are caused by the con- 

 struction of the eye-glasses of the telescope to which it is applied. If only one 

 eye-glass be used, the field is so contracted, that it is impossible to measure the 

 diameter of the sun or moon with precision, if the telescope magnifies above 30 

 times ; and if, to enlarge the field, we use the present construction of 1 eye- 

 glasses, the consequence is yet worse ; because equal spaces between the wires 

 will not then correspond to equal spaces on the objects it represents, as those 

 conversant in the theory of optics well know ; and this inequality depending on 

 the form, position, and refractive power of the first eye-glass, it will be impos- 

 sible to have data sufficiently exact to allow for that error. 



Those who were sensible of this defect have thought to correct it by the ap- 

 plication of an achromatic eye-glass, on the principle of that kind of object- 

 o-lass, not supposing it possible to correct the aberrations from the different re- 

 frangibility of light, and also from the spherical figure of the lenses by any 

 other means than combining a concave lens with the convex ones ; but the vio- 

 lent and contrary refractions from the necessary large size of the lenses, in pro- 

 portion to their focal lengths, not only occasioned great loss of light, but ren- 

 dered it impossible to correct the spherical aberration so as to obtain an angle of 

 vision much larger than could be had by a single eye-glass ; yet, however absurd 

 it may have appeared to attempt correcting both aberrations, when the lenses are 

 both convex, and are on the near sides of the wires, the following observations 

 will show the practicability of it, and may throw some light on the theory of 

 eye-glasses which seems hitherto not well understood. 



