0(36 Mr Herschel on Achromaik OhjecUGlasses, 
A 
tive or dispersive powers, 'at all likely to occur in practice. This 
remarkable circumstance affords a simple practical rule, appli- 
cable in all ordinary cases, for calculating the curvatures in any 
proposed state of the data, and requiring only the use of theo- 
rems with which every artist must be familiar, and, at all events, 
rendering it extremely easy to interpolate between calculated 
values. I have shewn in my paper, that a double object-glass 
will be nearly free from aberration^ provided the radius of the 
exterior surface of the crown lens be 6.72, and of the flint 14.2, 
the focal length of the comhinatim being 10.00, and the radii 
f the interior surfaces being comp^ited from these data, by the 
formulae given in all elementary worhs on optics, so as to mahe 
the focal lengths of the two glasses in the direct ratio of their 
dispersive powers. 
In this construction, the anterior glass, or that which first re- 
ceives the incident rays, is crown, and is double convex, of un- 
equal convexities, the flatter surface being placed outwards, 
while the posterior lens, formed of flint-glass, is concavo-convex, 
having its concave surface applied against the posterior or most 
convex surface of the crown lens. The combination is repre- 
sented in the annexed figure, where the four surfaces are num- 
bered in the order in which the light traverses them, O being 
the object, and F the image formed in the focus. 
The rule here stated is given only as approximative, and will 
no doubt be sufiiciently exact for ordinary use ; but when object- 
glasses of great size and value are to be constructed, their radii 
must be computed more strictly ; and^ for this purpose I have 
