THE 
EDINBURGH 
PHILOSOPHICAL JOURNAL. 
Art. I. — On the Practical Construction of Achromatic Object- 
Glasses. By Peter Barlow, Esq. E. R. S. Professor in 
the Royal Military Academy, Woolwich. Communicated 
' by the Author. 
i. A variety of methods have been proposed by mathemati- 
cians for determining the refractive and dispersive power of 
glass, and different principles have been given for computing 
the radii of curvature of the lenses composing the object-glass 
of our achromatic telescopes. The subject, indeed, is perhaps as 
well understood by theoretical opticians as can be desired ; but 
this is far from being the case with many who are practically 
engaged in the construction of telescopes, and for their conve- 
nience only the present article has been written. It professes to 
throw no new light upon this highly interesting subject, but 
merely to bring under one head, and to reduce to the most 
simple form, all that is actually required by the practical opti- 
cian, viz. 
1. To determine, in the most accurate manner, the refractive 
index of his two glasses : 
0. To determine their relative dispersive power: 
3. To determine the radii of curvature of the different sur- 
faces, so as to produce the achromatic property with the least 
spherical aberration. 
With a view to the former of these, the instrument recom- 
mended and described by M. Biot, in his Traite de Physique , 
VOL. XIV. NO. 07. JANUARY 1806. A 
