58 On the UNEQUAL 



of crown and flint glafe, ought to be one fixth of one fifty fifth 

 part of its aperture. The difference in the focal lengths of the 

 eye-glafTes will then render the indiftinclnefs nearly equal in 

 the two kinds of object- glafTes with equal magnifying powers, 

 in all cafes where their apertures are equal, and their lengths as 

 one to fix. 



There is, however, a circumftance of the greateft moment 

 to be taken into account before this conclufion can be admitted, 

 which is, that not merely the diameter of the circle of aberra- 

 tion is to be considered, but alfo the fpiflitude of the rays, both 

 within that circle in general, and at different dillances from its 

 centre. The rarity of the light in the fimple fpeclrum is fuch, 

 that the aberration hurts much lefs than might be expected. 

 But in the fecondary fpeclrum, a6 two orders of coloured light 

 are united, the imperfect union of the rays by the compound 

 object- glafs, will hurt the diftinctnefs much more, in propor- 

 tion to the extreme divergency. 



On this account, it is to be expecled, that the proportional 

 lengths of the fpectrums, when the experiment comes to be pro- 

 perly made, will turn out lefs than as one to fix, notwithstand- 

 ing the degree in which the diftinctnefs is hurt in the two kinds 

 of telefcopes, from the unequal refrangibility of light, may be 

 nearly in that proportion. 



The principal improvement of refracting telefcopes, pointed 

 out by the preceding experiments, confifts in an entire removal 

 of this aberration from the unequal refrangibility of light. It 

 appears from the performance of the fmall telefcope above men- 

 tioned, in which the fecondary colour is not removed, that con- 

 fiderable advantages may alfo be expected from fubflituting a 

 more perfect medium for flint-glafs ; from a more perfect cor- 

 rection of the aberration from the fpherical figure ; from pre- 

 venting that lofs of light by reflection, which takes place when 

 light enters into, or emerges from denfe mediums furrounded 



with 



