TELESCOPE. 



Example 5 Let the ratio of the radii of the convex lens 



be I : 6, in which tlie fpherical aberration is a m'inimum, and 

 let the crown and flint glafs be as in the iirfl example for a 

 telefcope of 30 inches ? ' 



Then, b-y the fame procefs, there will come out 



R =3l!92} ^ = 9-12. as before. 



'R = I2.6ol ,T- , r 



''■ = 15-50 J ^ '^■^' '" 



Tiiis table is proper for glafs of the fame refraftive and 

 difperiive powers as in Table I., but with the fpherical 

 aberrations the leaft poflible. 



Table V — Radiiof double Objeift-glafTes in Inches, where 

 the Convex has a Minimum of Aberration. 



Table VI. — Radii of double Objeft-glalTes in Inches. 



In the preceding example, where the ratio of the two 

 radii of the convex lens were given, r comes out =: 5.32 in 

 a thii-ty-inch telefcope ; and in like manner, when the aber- 

 ration only is given, the ratio of the radii may firft be de- 

 termined by a quadratic equation, and then the other curves 

 may be determined as they have been here, without any af- 

 fumption of r in the convex lens. But in all cafes the re- 

 lative refraftive and difperfive powers muft be known pre- 

 vioufly to the calculations fuch as we have exemphfied. 



Example 6. — In this example let us take the fame crown 

 and flint glafs as we did in the fecond example, atid put 

 r = 7.5, as in our firft example ; and then the radii will be 

 as in the following table. 



Example 7 Let us take in this example the crown and 



flint glafs as in the firft example, and put ;• = 9, as in our 

 fecond example, and then the radii will come out as in the 

 fubjoined table. 



Table VII — Radii of double Objea-glaffes in Inches. 



