I recently found a wonderful article while browsing old public domain articles – Philosophical Transactions of the Royal Society from the late 17th century. Here’s Laurent Cassegrain, presenting his telescope design in 1672:
The advantage, which I find in this Instrument above that of Mr. Newton, is first, that the mouth or aperture AB of the Tube may be of what bigness you please; and consequently you may have many more rays upon the Concave Speculum, that upon that, of which you have given the description.
- The reflexion of the rays will be very natural, since it will be made upon the axis it self, and therefore more vivid.
- The vision of it will be so much the more pleasing, in that you shall not be incommoded by the great light, by reason of the bottom CD, which hideth the whole face.
Besides that you’ll have less difficulty in discovering the Objects, than in that of Mr. Newtons.
Isaac Newton responds:
I should be very glad to meet with any improvement of the Catadioptrical Telescope; but that design of it, which (as you informe me) Mr. Cassegrain hath communicated … I fear will not meet expectation. For, when I first applied myself to try the effects of Reflexions … I had thence an occasion of considering that sort of construction, and found their disadvantages so great, that I saw it necessary, before I attempted any thing in the Practique, to alter the design of them, and place the Eye glass at the side of the tube rather than at the middle.
He then proceeds to give seven technical reasons why the entire design is totally flawed, before concluding with:
By this you may percieve, that the three advantages, which Monsieur Cassegrain propounds to himself, are rather disadvantages. For, according to his design, the aperture of the instrument will be but small, the object dark and confused, and also difficult to be found … You see therefore, that the advantages of this design are none, but the disadvantages so great and unavoidable, that I fear it will never be put in practice with good effect.
And finally, just to finish it off: “I could wish therefore, Mr. Cassegrain had tryed his design before he divulged it: but if, for further satisfaction, he please hereafter to try it, I believe the success will inform him, that such projects are of little moment till they be put in practice.”
If you’re interested, the article in question can be found on JSTOR or in this bittorrent archive with all the papers published in the Transactions from about 1665 up to 1923, which are thus in the public domain. Note: I’ve kept the formatting in the quotes as in the original, with the exception that I’ve changed the long s “ſ” to a normal s.
