44 On the UNEQUAL 
concave lenfes of this kind, the farther contrivance of a: ibsranni 
ring to confine the fluid is required.. 
Tue effect of the above votabinition: which was of bat of 
turpentine with a mineral oil, I immediately perceived to be 
different from what was obferved in the preceding trials... The 
green and purple fringes {till appeared, and they lay in the fame 
order as before ; but their breadth was greatly diminifhed, I 
judged about one half. 
Tuis new fact was the only fruit of this laft. fet of experi- 
ments, which were attended with much trouble and Jlofs of 
time. For to make them with the requifite degree of preci- 
fion, pains muft be taken, not only to get the refraction as’co- 
lourlefs as the qualities of the mediums will admit; but alfo:to 
compute the error from the fpherical figure, and. procure lenfes 
accurately ground to the {pheres which: are required.’ Unlefs 
thefe points are duly attended to, accuracy in the refults is not 
to be expected. 
I wow: confidered how this Berdbaace of the breadth “of 
the coloured fringes, obferved in the laft mentioned experiment, 
might beft be turned to account. In the firft place, it was ob- 
vious, that an objeét-glafs, formed by a combination of the 
mediums ufed in that experiment, would have an advantage 
over others, in which the correction of the aberration from the 
difference of refrangibility is more imperfe@t. But as this 
fault, though greatly diminifhed, would ftill prevent the ufe of 
high magnifying powers, I weighed the circumftances more 
attentively, and the matter appeared to me in the meet : 
light : 
A convex lens, formed of the leaft difperfive of the two 
effential oils, being fo combined with a concave lens, formed 
of that which is moft difperfive, as to unite the red and violet 
rays, leaves fringes of uncorrected colour, much narrower 
than thofe produced by compound object-glaffes of the fame 
focal diftance, formed by a combination of either of thefe fluids 
with 
