We cannot conclude this account of the optical la bours of Huygens, without noticing his theory of light, which Ile has explained at great length in his 7'raite de la Lumiere already mentioned. This treatise is di vided into six chapters, the first of which treats oflight propagated directly ; the second of reflexion ; the third of refraction ; the fourth of the refraction of the air ; the fifth of Iceland crystal ; the sixth of the figures of trans parent bodies, for the purposes of refraction and re flexion. The fifth chapter occupies nearly one half of the treatise; but in the first chapter he explains his theory of light, which he supposes to be produced in the same manner as sound, by undulations propagated in an elastic ethereal medium ; and he shows how, by this principle, all the phenomena of refraction and re flexion may be produced. This hypothesis was after wards revived by the celebrated Euler, and ably de fended and illustrated by our countryman Dr. Thomas Young. The law ol interference, discovered by this distinguished pilosopher, and other recent discoveries, have drawn to it the attention of many eminent modern philosophers, and have gained !or it a degree of support which threatens to overwhelm the Newtonian doctrine of emission.
While the sciences of optics and astronomy were ad vancing hand in hand on the contilient, in consequence of the improvements which were making on the refract ing telescope, our countryman James Gregory enriched the science, by his invention of the reflecting telescope, which is described at the end of his Optira Promota, published in 1663. It consisted of a parabolic concave mirror, perforated at the ceutre, it, froot ol which was placed a small concave elliptic speeuluni, at a distance a little greater than the sum of their local lengths. The parallel rays issuing- from a distant object formed an image in front ol the great mirror, in its focus; and this image, serving as another ohject, had ait image ol itself formed behind the great mirror, by the small One, by means of rays passing timing') the perforation in its centre. This image was then viewed with an eye-glass. The invention of this instrument was followed, in 1666, by the Newtonian telescope, which consisted of an un perforated mirror, that threw the image of the object upon a small plane speculum, placed at an angle of 45° within its focus, so as to reflect the image aside to the eye of the observer, where it was viewed with an eye glass. About the year 1672, Al.Cassegrain gave a new fora, to the reflecting telescope, by using a convex small speculum, placed nearer the great speculum than its principal focus. The effect of this change was to shorten the telescope ; and there is reason to think that this instrument gives more perfect vision than those of either the Gregorian or Newtonian form.
As is the case with all remarkable inventions, Gre gory has riot been allowed to maintain the undisturbed possession of the honour of having invented the reflect ing telescope. Father :Nlersenne,* in a correspondence with Descartest in 1639, but which was not printed till 1666, three years after the appearance of the °Mira Pronzota, proposed to Descartes the idea of using con cave mirrors in the construction of telescopes ; but, so far from having invented and constructed an instrument of value, Descartes endeavoured to convince him of the fallacy of his views. Another claimant for the same honour was brought forward by Fontenelle, who, in his history of the Academy of Sciences for 1700, has ven tured to say, " that the reflecting. telescope was begun to be used as early as the year 1 6 1 6." " Father Zuc chi," he continues, " an Italian Jesuit, in his °Mira Philosolzhia, printed at Lyons in 1652, says, that in 1616, reflecting on the theory of telescopes then recently in vented, it came into his head to employ concave metal lic mirrors, znstead of object IC7ISCC qf glass, in order to produce by rcflexion the same effects which result from refraction. Having found, therefore, in a cabinet of cu riosities, a concave metallic mirror, exactly worked by an able artist, he applied to it a concave eye-glass., and with this telescope observed terrestrial and celestial objects ; and experience conlirmed what tocory had taught Itim.".1 We need scateely appeal to the candour of the reader for his decision upon this claim. Zue.chi obviously made an expel imeot with a concave mirror and a con cave lens, in Li1C matmer as we ilZsile IT3St)11 to believe was done by Roger Baton and Leonaro Dwges. As there was no small speculum, the tete at ope 116. ther Gregorian, Newtonian, nor Cassega.,iniali ; and can be considered hi no other light than a lud anti imper fect experiment by oblique reflexion, which must ;rave distorted and spoiled the image, even it ais speculum had possessed a true figure. Had 'Gucci,' told us the local length of las mirror, we might probably have found it to be so short, that the idea ol the ex periment a telescope, might nave been still more ridi culous thati it appears to be, when we are left to form any notion we please of the magnitude and local distance of the speculum. If Zucchi had conceived himscli the inventor of the refit cting telescope. why did he remain contented with a niiiror lound by accident in a cabinet of curiosities ? Ile mos' have thought his experiment useless, otherwise he WOUlti not have failed to improve it by the addition of a tube, and the application ol a small speculum.