70 On the UNEQUAL 



The application of the demonflrations of Hu genius to the 

 correction of the aberration from the fpherical figures of lenfes, 

 whether folid or fluid, is then taken notice of, as being the next 

 flep towards perfecting the theory of telefcopes. 



Next it appears from trials made with objecl-glafTes of very 

 large apertures, in which both aberrations are corrected as far 

 as the principles will admit, that the correction of colour which 

 is obtained by the common combination of two mediums 

 which differ in difperfive power, is not complete. The homo- 

 geneal green rays emerge mofl refracted, next to thefe the uni- 

 ted blue and yellow, then the indigo and orange united, and 

 laftly the united violet and red, which are leaft refracted. 



If this production of colour were conftant, and the length of 

 the fecondary fpectrum were the fame in all combinations of 

 mediums when the whole refraction of the pencil is equal, the 

 perfect correction of the aberration from difference of refrangi- 

 bility would be impoffible, and would remain an insurmount- 

 able obftacle to the improvement of dioptrical inftruments. 



The object of the next experiments is, therefore, to fearch, 

 whether nature affords mediums which differ in the degree in 

 which they difperfe the rays compofing the prifmatic fpectrum, 

 and at the fame time feparate the feveral orders of rays in the 

 fame proportion. For if fuch could be found, the above men- 

 tioned fecondary fpectrum would vanifh, and the aberration 

 from difference of refrangibility might be removed. The re- 

 fult of this inveftigation was unfuccefsful with refpect to its 

 principal object. In every combination that was tried, the 

 fame kind of uncorrected colour was obferved, and it was 

 thence concluded, that there was no direct method of remo- 

 ving the aberration. 



But it appeared in the courfe of the experiments, that the 

 breadth of the fecondary fpectrum was lefs in fome combina- 

 tions than in others, and thence an indirect way opened, lead- 

 ing to the correction fought after ; namely, by forming a com- 

 pound 



