*6 d Mr, ramsden m a new Conjlruclion 
brought to the prifm, the lefs will be- tire -bo rd re of colours 
i on the contours of the objeCd. 
To apply this to practice I placed a piano convex lens a {vide 
fig. i.) with its plane fide near an object, or an image IN formed 
by the objeCt-glafs of atelefcope, and thus magnified the image 
'which, from the pofition of the lens, was fenfihly free from 
colour ; but the relpeCtive foci of a lens fo placed being very 
: near each other, and on the fame fide, the emergent pen- 
*- cils diverge - on .-the eye, -and give indiftinCt vihon : this 
was remedied by placing a fecond lens k a little within the 
focus of ' the v former, the combined foci of the two lenfes 
being in the place of the image, the rays were thereby made 
to tall parallel on the- eye, and to Ihew the objedt IN difiinCfly. 
If, by putting the lens a very near th emu age, any imperfection 
;.in it becomes too vifible, that difiance may be confiderablv in- 
oreafed, without producing any bad effeCt ; for theory, as wellas 
' experiment, thews, that a fmall aberration from the different 
refrangibility of light is of little confequence compared with 
■the fame quantity. of aberration caufed by the fpherical figure 
* of the lenfes, but even that colouring may be correCfed in the 
nearer eye-glafs-*. for let a ray (fig. 2.) from an objeCt 0, by 
«.pafling through a lens B, be feparated into colours, ac being 
the direCfion of the violet rays, and at that of the red; if 
another lens be put at c, the violet rays pafiing through its 
• center will fuffer no refrattion, while thofe of the red, pafiing 
.at fome diftance from thence are refraCted, and the emergent 
red and violet will be parallel, when the mean refraCting 
angle of the lenfes at the incidence of each pencil are to one 
another inverfely as the diameters of thofe pencils. 
If we attend to this pofition of the eye-glafifes, it will be 
found equally advantageous for obviating the fpherical aberration 
©f 
