REFRANGIBILI1 r of LIGHT. . 59 



with air ; and from diminifhing thofe errors which arife from 

 faults in the workmanfhip. 



The difad vantages under which reflecting telefcopes labour, 

 arife from their requiring larger apertures to tranfmit the fame 

 quantity of light ; from being found to be more affected by 

 imperfections of the atmofphere than refracting telefcopes, and 

 being liable to tarnifh ; but [principally from imperfections in 

 the workmanfhip of the object fpeculum hurting their perform- 

 ance much more than equal imperfections in the object-glafs 

 hurt refractors. 



The deviation of a ray from its intended courfe, occafioned 

 by an imperfection in the figure of a reflecting fpeculum, is to 

 its deviation,, arifing from an equal imperfection in a lens, as 

 four to one, when the ray panes from glafs into air, and in the 

 proportion of fix to one, when it pafTes from air into glafs. At 

 a medium, therefore, it may be flated as five to one. It follows 

 from hence, that fuppofing all other caufes of imperfection re- 

 moved but this of workmanfhip, and that the metal of fpecu- 

 lums were capable of as good a polifh as glafs, and of reflecting 

 as much light as glafs tranfmits, ftill the perfection of the 

 images of objects formed by refraction would greatly exceed 

 thofe by reflection. 



Such is the cafe in the refractions which take place in the 

 confine of glafs and air. But in the refractions made in the 

 confine of glafs, and mediums of greater denfity than air, the 

 difference is ftill much greater. 



The proportion of the fine of the angle of incidence to the 

 line of the angle of refraction of a ray in patting out of one 

 medium into another medium, is compofed of the proportion 

 of the fine of the angle of incidence to the fine of the angle of 

 refraction out of the fir ft medium into any third medium, and 

 of the proportion of the fine of the angle of incidence to the 

 fine of the angle of refraction, out of that third medium into 

 the fecond medium. 



H 2 Thus, 



