44 On the UNEQUAL 



concave lenfes of this kind, the farther contrivance of a glafs- 

 ring to confine the fluid is required. 



The effect of the above combination, which was of oil 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 ftill appeared, and they lay in the fame 

 order as before ; but their breadth was greatly diminifhed, I 

 judged about one half. 



This new fact was the only fruit of this laft fet of experi- 

 ments, which were attended with much trouble and lofs of 

 time. For to make them with the requiiite degree of preci- 

 fion, pains mud 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 fpheres which are required. Unlefs 

 thefe points are duly attended to, accuracy fax the refults is not 

 to be expected. 



I now confidered how this diminution 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 object-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 imperfect. 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 following 

 light : 



A convex lens, formed of the lead difperlive 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 



