Mr. ramsden on a new ConJiruSlion , c5V. (/5 
times ; and if, to enlarge the field, we ufe the prefent con- 
ftruCtion of two eye-glafles, the confequence is yet w orfe ; 
becaufe equal fpaces between the wires will not then correfpond 
to equal fpaces on the objects it reprefents, as thofe conver- 
fant in the theory of optics well know ; and this inequality 
depending on the form, pofition, and refractive power of the 
firft eye-glafs, it will be impoffible to have data fufficiently 
exaCt to allow for that error. 
Thofe who were fenlibie of this defect have thought to correct 
it by the application of an achromatic eye glafs, on the prin- 
ciple of that kind of objeCt-glafs, not fuppofing it pofli- 
ble to correct the aberrations from the different refran- 
gibility of light, and alfo from the fpherical figure of 
the lenfes by any other means than combining a concave lens 
with the convex ones ; but the violent and contrary refractions 
from the neceffary large lize of the lenfes in proportion to their 
focal lengths, not only occafioned great lofs of light, but 
rendered it impofiible to correCt the fpherical aberration fo a,s to 
obtain an angle of vifion much larger than could be had bv a 
fingle eye-glafs : yet, however abfurd it may have appeared to 
attempt correcting both aberrations, when the lenles are both 
convex, and are on the near fide of the wires, the following 
obfervations will fhew the practicability of it, and may throw 
fome light on the theory of eye-glafl'es which fee ms hitherto 
not well underftood.. 
Sir Isaac newton has (hewn in his Leulione s Optics, in that 
ffCtion De Phcenomenis lucis per prifmain OcuJum tranfmtjja , that 
the appearance of colours on the edges of objeCts when viewed 
through a prifm depends on the proportion pf the diftance be- 
tween the prifm and the objeCt, compared with that between 
the prifm and. the eye, that is to lay, the nearer the objeCt is 
brous ht 
