and Meteors, as well as passages in the letters. His optical investigations are perhaps the subject in which he most contributed to the progress of science ; and the lucidity of exposition which marks his Dioptrics stands conspicuous even amid the generally luminous style of his works. Its object is a practical one, to determine by scientific considera tions the shape of lens best adapted to improve the capa bilities of the telescope, which had been invented not long before. The conclusions at which he arrives have not been so useful as he imagined, in consequence of the mechanical difficulties. But the investigation by which he reaches them has the merit of first prominently publishing and establishing the law of the refraction of light. Attempts have been made, principally founded on some jealous remarks of Huyghens, to show that Descartes had learned the principles of refraction from the manuscript of a treatise by Willebrord Snell, but facts do not bear out the charge ; and, so far as Descartes founds his optics on any one, it is on the researches of Kepler. In any case the glary of the discovery is to a large extent his own, for his proof of the law is founded upon the theory that light is the tendency or inclination of the subtle particles of ethereal matter to propagate their movement in straight lines from the sun or luminous body to the eye. And thus he approxi mates to the wave theory of light, though he supposed, like his contemporaries, that the transmission of light was instantaneous. The chief of his other contributions to optics was the explanation of the rainbow an explanation far from complete, since the unequal refrangibility of the rays of light was yet undiscovered but a decided advance upon his predecessors, notably on the De radiis visus et lucis (1611) of Marc-Antonio de Dominis, archbishop of Spalato, from whom careless critics have assumed that he derived his ideas. If Descartes had contented himself with thus explaining the phenomena of gravity, heat, magnetism, light, and similar forces by means of the molecular movements of his vortices, even such a theory would have excited admiration by its daring grandeur. But Descartes did not stop short in the region of what is usually termed physics. Chemis try and biology are alike swallowed up in the one science of physics, and reduced to a problem of mechanism. This theory, he believed, would afford an explanation of every phenomenon whatever, and in nearly every department of knowledge he has given specimens of its power. But the most remarkable and daring application of the theory was to account for the phenomena of organic life, especially in animals and man. " If we possessed a thorough know ledge," he says, 1 " of all the parts of the seed of any species of animal (e.g., man), we could from that alone, by reasons entirely mathematical and certain, deduce the whole figure and conformation of each of its members, and, con versely, if we knew several peculiarities of this conforma tion, we could from these deduce the nature of its seed." The organism in this way is regarded as a machine, con structed from the particles of the seed, which in virtue of the laws of motion have arranged themselves (always under the governing power of God) in the particular animal shape in which we see them. The doctrine of the circulation of the blood, which Descartes adopted from Harvey, supplied additional arguments in favour of his mechanical theory, and he probably did much to popularize the discovery. A fire without light, compared to the heat which gathers in a haystack when the hay has been stored before it was properly dry heat, in short, as an agitation of the particles is the motive cause of the contraction and dilatations of the heart. Those finer particles of the blood which become extremely rarefied during this process 1 (Euvres, iv. 494. pass off in two directions one portion, and the least important in the theory, to the organs of generation, the other portion to the cavities of the brain. There not merely do they serve to nourish the organ, they also give rise to a fine ethereal flame or wind through the action of the brain upon them, and thus form the so-called " animal " spirits. From the brain these spirits are conveyed through the body by means of the nerves, regarded by Descartes as tubular vessels, resembling the pipes conveying the water of a spring to act upon the mechanical appliances in an artificial fountain. The nerves conduct the animal spirits to act upon the muscles, and in their turn convey the im pressions of the organs to the brain. Man and the animals as thus described are compared to automata, and termed machines. The vegetative and sensitive souls which the Aristotelians had introduced to break the leap between inanimate matter and man are ruthlessly swept away ; only one soul, the rational, remains, and that is restricted to man. One hypothesis supplants the various principles of life ; the rule of absolute.mechanism is as complete in the animal as in the cosmos. Reason and thought, the essential quality of the soul, do not belong to the brutes ; there is an impassable gulf fixed between man and the lower animals. The only sure sign of reason is the power of language i.e., of giving expression to general ideas; and language in that sense is not found save in man. The cries of animals are but the working of the curiously-contrived machine, in which, when one portion is touched in a certain way, the wheels and springs concealed in the interior perform their work, and, it may be, a note supposed to express joy or pain is evolved ; but there is no consciousness or feeling. " The animals act naturally and by springs, like a watch." 2 "The greatest of all the prejudices we have retained from our infancy is that of believing that the beasts think." 3 If the beasts can properly be said to see at all, " they see as we do when our mind is distracted and keenly applied elsewhere ; the images of outward objects paint themselves on the retina, and possibly even the impressions made in the optic nerves determine our limbs to different movements, but we feel nothing of it all, and move as if we were automata." 4 I will not believe, said the Cartesian Chanet. that a beast thinks until the beast tells me so itself. The sentience of the animal to the lash of his tyrant is not other than the sensitivity of the plant to the influences of light and heat. It is not much comfort to learn farther from Descartes that " he denies life to no animal, but makes it consist in the mere heat of the heart. Nor does he deny them, feel ing in so far as it depends on the bodily organs." 5 Descartes, with an unusual fondness for the letter of Scrip ture, quotes oftener than once in support of this monstrous doctrine the dictum that " The blood is the life ;" and he remarks, with some sarcasm possibly, that it is a comfort able theory for the eaters of animal flesh. And the doctrine found acceptance among some whom it enabled to get rid of the difficulties raised by Montaigne and those who allowed more difference between animal and animal than between the higher animals and man. It also encouraged vivisection a practice common with Descartes himself. 8 The recluses of Port Royal seized it eagerly, discussed automatism, dissected living animals in order to show to a morbid curiosity the circulation of the blood, were careless of the cries of tortured dogs, and finally embalmed the doctrine in a syllogism of their logic, No matter thinks ; every soul of beast is matter : therefore no soul of beast thinks. But whilst all the organic processes in man go on 8 (Euvres, ix. 426. * (Euvres, vi. 339. 6 (Euvres, iv. 452 and 454. 3 (Euvres, x. 204. 6 (Euvres, x. 208.
