OF FLUID REFRACTING TELESCOPES. 
13 
I have therefore made the length twelve feet, to an aperture of eight inches, 
which, although shorter than opticians would choose to work in the usual 
achromatic, is not so short as this principle of construction would admit, and 
which in any new case I should not hesitate to adopt. Indeed, according to 
the form of construction I am now about to propose, a telescope of two feet 
aperture and twenty-four feet in length would not have more spherical aber- 
ration to contend with, than a telescope of the usual construction of six inches 
aperture and twelve feet length, which is fully within the range of the usual 
practice ; at the same time I will not undertake to say that I could on so large 
a scale confine the length to twelve times the aperture, although I should 
certainly attempt it in the first instance. But if the length extended to even 
fifteen or eighteen times the aperture, I have little doubt of making the instru- 
ment manageable by one person, by adequate mechanical arrangements, and 
of producing a telescope which would as much exceed the most powerful tele- 
scopes of the present day, as these exceed the refractors of highest repute at 
the close of the last century. 
Whether such an instrument will be undertaken at present, depends upon 
circumstances which I cannot command. I can only say, that if such a con- 
struction were entrusted to my direction, no exertion should be wanted on my 
part, to render it complete and worthy of the present state of English science. 
At all events I cannot doubt that the spirit of scientific enterprise will lead 
ultimately to the attempt ; and in order to facilitate the accomplishment of 
it, as far as lies in my power, I have in the following pages described the 
nature of the arrangements which in my opinion would most contribute to 
success. 
In my former paper I have given a formula expressing the relations between 
the length, foci, and distances of the lenses, and have remarked upon the 
almost infinite variety of forms to which it leads ; some of them, I have stated, 
would probably be found in practice preferable to others, although they are all 
equally correct in theory. Of these cases, some have since suggested them- 
selves to me ; and others will also probably be detected, by a due examination 
of the formula and tables, which Professor Littrow, of Berlin, has recently 
presented to the Astronomical Society, relative to this form of telescope ; with 
tables of curvatures, both direct from the formulae of Euler (reduced to the 
