311 
Professor Gautier on the Comet of July 1825 s 
the whole of the observations of this comet taken together, will 
permit our assigning it a very elongated elliptical orbit. Lastly, 
it is possible that its proximity to the earth may have produced 
some influence upon its elements, from the perturbations that may 
have resulted from it. M.Plana has had the goodness to promise 
to send me the observations which he made on the 6th and 17th 
of October, which may serve to rectify my elements. I have 
judged it proper, however, not to delay the communication of 
my first results, hoping that they may serve at least to give an 
idea to those who are not familiar with the theory of comets, of 
what may be deduced in an approximative manner by means of 
three observations only. 
P. There is to be found in the 5th Number of the 13th 
Volume of the Correspondance Astronomique, an ephemeris of 
the comet by M. Capocci, and of M. Hansen’s elliptical elements, 
according to which this comet would make its revolution about 
the sun in 382 years. 
Art. XIII. — On the Practical Construction of Achromatic Ob- 
ject-Glasses. By Peter Barlow, Esq. F. B. S. Professor in 
the Royal Military Academy, Woolwich. Communicated 
by the Author. (Continued from Vol. XIV. p. 18). 
We may now proceed to the calculation of the radii for a com- 
pound achromatic object-glass, the indices of refraction, and the 
dispersive power of the glasses being given. 
15. Detail of the computation for a compound Achromatic Ob- 
ject-Glass. 
It is best to make the calculation, in this case, always for a 
given compound focal length, and afterwards to alter the curva- 
tures in the direct ratio of the proposed focal length to that 
assumed. Our assumed compound focal length is always 10 
inches. 
The example we shall propose is to compute the curvatures 
of a compound object-glass, made from the two specimens of 
plate and flint experimented upon, as in the leading part of this 
paper. The index of the plate being 1.528, of the flint 1.601 ; 
3 
