314 
Mr Barlow on the Practical Construction 
No. in 4th column — 4 - 2*45 
Multiplied by *016 
Gives 
Add 
Sum 
Add tab. rad. 
4 - -03920 cor. pi. index. 
4- *01656 cor. fl. index. 
4- *05576 wh. correction. 
6.7956 
Corrected radius 6-8514 for the first surface. 
Again, No. in 6 th column = + 116-14 
Multiplied by -004 
Gives 
And, No. in 7th column = 
Multiplied by 
Gives 
4 - ‘46456 cor. pi. index. 
71-69 
•016 
— 1*14704 cor. fl. index. 
+ *46456 
Difference 
Tab. rad. 
— *68248 wh. correction. 
12-7422 
12-0597 corrected radius for 
the 4th surface. 
It now only remains to find the 2d and 3d, or the contact 
surfaces. For this determination we have given for each lens 
the focal length, the radius of one surface, and the index of re- 
fraction, which, therefore, for the double convex or plate lens, 
fall under rule 6 of article 14, and for the flint lens, which is 
concavo-convex, under rule 8 of the same article. 
For 2d surface plate lens, (See rule 6 page 16.) 
Focal length = 3-17, Decimal part of index — *528 ; 
3-17 X ‘528 — 1.67376 First product 
rad. First surface = 685* 
First prod x rad. 1st surface =: 11.4627 = dividend. 
6-85 — 1-67376 = 5-17 = divisor, 
5*17) 11-4627) 2-22 = 2 d surface. 
