Grusp—On a New Form of Equatorial Telescope. 63 
of as a “ Dialyte” form of telescope I would gain a considerable advantage. To 
explain this : figure 2 represents a section of the cone of rays in the case of an ordi- 
nary achromatic telescope, and figure 3 that of the cone of rays in a dialyte, in 
which case the achromatism is corrected, not by a flint lens close to the crown, but 
by a flint, or combination of flint and crown, having a negative power, at a 
considerable distance from the crown. 
Now if these two figures be examined, it will be at once seen that the section of 
the cone of rays at any given distance from the objective (say, one-third of the 
whole focus) is much smaller im the case of the dialyte than the ordinary form, and 
therefore a smaller mirror would suffice. 
I have lately had an opportunity of examining one of those dialytes, made by 
Pléssel, and presented by a late Emperor of Austria to the Vienna Observatory, 
Fig. 2. 
Fig. 3. 
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and it appears to me that the corrections can be made exceedingly perfect for the 
centre of the field. 
The definition at the edge of the field, however, is not so good as in the ordi- 
nary form; but this would not be of so much consequence in large instruments, as 
the field in such cases is never of great extent. 
. The dialyte form of telescope was originally devised to meet the exigencies of 
a time when it was much easier to get large pieces of crown than of flint glass ; 
but of late years it has been more patronized by amateurs than by professional 
opticians, and for this reason: in the ordinary forms of objectives the curves have 
to be very accurately calculated and figured, in order to correct both for chromatic 
and spherical aberration ; but in the dialyte, given any three lenses, a large crown, 
and a small pair of crown and flint only very approximately right in foci and form, 
and a good dialyte can be made out of them by modifying the various positions 
K 2 
