18 
Mr Barlow On Achromatic Object Glasses . 
9 . The index of refraction and radius of concave surface being given ? 
to find the radius of the convex surface , so that the lens may have 
a given negative focal length. 
Rule. — Find the dividend exactly as in Rule 6. Then sub- 
tract the given radius from the first product for a divisor. Di- 
vide the dividend by the divisor, and it will give the radius re- 
quired. 
Example . — Let the numbers stand as in the last example, 
except that the given concave radius is 2*4 inches, and let the 
other radius be computed. 
First product — 3*0 
Given radius, — 2-4 
3*0 — 2-4 - 6) 72 
Radius sought, = 12 inches. 
Remark. In a similar manner, the radii of curvature of a 
lens being given, and its focal length found by experiment, its 
index may be computed with great accuracy. On this subject 
some observations will be found in the concluding part of this 
paper. 
( To he continued.) 
Art. II. — General Reflections on various important subjects in 
Mineralogy. By Frederick Mohs, Esq., Knight of the 
Order of Civil Merit, Professor of Mineralogy at Freyberg, 
Fellow of the Royal Society of Edinburgh, of the Wer- 
nerian Natural History Society, he. Continued from VoL 
XIII. p. 218. 
We shall not in this place inquire what kind of information, 
relative to the products of the mineral kingdom, should be ex- 
cluded from Mineralogy ; but it is necessary for us to examine 
whether, by proceeding solely upon the observation and compa- 
rison of the natural-historical properties of simple minerals, 
we may arrive at something which, besides containing informa- 
tion of one and the same kind throughout, also possesses the 
