168 Mr. Lubbock on the Double Achromatic Object Glass. 



In this construction the thickness of the second lens may 



also be made inconsiderable, and as - A -r— and — K — 



A 4 m'A 3 A 4 A 3 



are small quantities, (see the numerical values, p. 167,) t 2 

 may certainly be disregarded in the coefficient of y 2 in the ex- 

 pression for — . 



Neglecting y* and — , 



*-i~<~<-*}-3» 



S m 

 m' — 1 



In the numerical example given, p. 166, supposing the aper- 

 ture = 1, and the thickness of the double-convex lens to 

 vanish at the circumference of the field, t x — '06 nearly. 



/a* ~ («'+2) ^ 4 +f*)fa + ^rpg \f* + f* (/4 +/a) } 



m*(m'-l) 2 € / 1 1 \ 



+ (/4-/ 2 )(^ + 2)^>-l) 2 t Ua « (*i + 'i)/ 



VSJ ~" An + tJ + wA, (A + *,)*/ == °* 

 I find, (retaining the previous values of r lt r^) see p. 166, 

 that is, making r, = —6*7069 r 9 = 3*0488 



1 



A, 



= — -051265) 

 A = jr = -'25024 / 4 = i = -'10023 



