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resolution of the question, whether light is accelerated or retarded on passing from a rarer to a denser medium. *
On this reasoning Professor Robison has very well remarked, that it would be just, if the light, on entering the water telescope, had only its velocity changed, and not its direction. But this is
not the case; for the ray that is to go down the axis of the telescope, is not perpendicular to the surface of the fluid; it makes an angle with it, depending on the aberration, and therefore in some cases less by 20" than a right angle. On this account, the effect is not produced which Boscovich's reasonings lead us to expect.
The sequel of the paper is also full of ingenious remarks.
In December 1785, Mr Robison was attacked by a severe disorder, which, with but few intervals of relaxation, continued to afflict him to the end of his life, and which, though borne with much resignation, and resisted with singular fortitude, could not but at length impair both the vigour and the continuity of his exertions. The disorder seemed to be situated between the urethra and the perineum. At times it was accompanied with the severest pain, and with violent spasms, which were easily excited. The disease, however, was only known by the pain pro
* Boscovich, Opera Math. Tom. II. Opusc. 3.
duced; and never, by any visible or palpable symp tom, gave information of its nature, as no change in the parts which were the seat of it could ever be observed. A complaint of this nature, it is evident, must have less chance of being removed than any other; and it accordingly baffled the art of the most skilful medical men, both in Edinburgh and London.
Notwithstanding this state of suffering, his gene. ral health was not, for a long time, materially injured, nor the powers of his mind relaxed, so that he continued to prosecute study with vigour and steadi. ness. A malady which was both severe and chroni cal, admitted of no palliative so good as the comfort of domestic society, which Mr Robison happily enjoyed, having married soon after he settled in Edinburgh. The care and attention of Mrs Robison, and the affectionate regards of his children, as they grew up, were blessings to which, with all his habits of study and abstraction, he was ever perfectly alive.
This indisposition did not prevent him from engaging, about this time, in a very laborious undertaking. A work, with the title of Encyclopædia Britannica, undertaken at Edinburgh several years before this period, was now undergoing a third edition, in which it was to advance from three to eighteen volumes. Twelve of these had been al ready published, under the direction of the original
editor, Mr Colin Macfarquhar, when, on his death, the task of continuing the work was committed to the care of the Reverend Dr Gleig; and about the same time Professor Robison became a contributor to it. He was the first contributor who was professedly and really a man of science; and from that time the Encyclopædia Britannica ceased to be a mere compilation. Dictionaries of arts and sciences, in this island, had hitherto been little else than compilations; and, though in France, the co-operation of some of the most profound and enlightened men, of the age had produced a work of great merit and celebrity, with us compositions of the same class had been committed to the hands of very inferior artists. The accession of Professor Robison was an event of great importance in the history of the above publication.
It was in the year 1793 that he began to write in this book, and it was at the article Optics, with him a very favourite science, that his labours commenced. From that time he continued to enrich the Encyclopædia with a variety of valuable treatises, till its completion in 1801.
The general merit of the articles thus composed makes it difficult to point out particulars. Those in which theoretical and practical knowledge are combined are of distinguished merit; such are Seamanship, Telescope, Roof, Water-works, Resistance of Fluids, Running of Rivers. To these I
must add the articles Electricity and Magnetism in the Supplement, where the theories of Epinus are laid down with great clearness and precision, as well as with very considerable improvements. In ascertaining the law of the electric attraction, his experiments were ingenious, as well as original, and afforded an approximation to the result which the great skill and the excellent apparatus of Coulomb have since exactly ascertained. In the Supplement is also contained a very full account of the Theory of Boscovich, a subject with which he was much delighted, and which he used to explain in his lectures with great spirit and elegance.
These articles, if collected, would form a quarto volume of more than a thousand pages. I am persuaded, that, when brought together, and arranged by themselves, they will make an acceptable present to the public; and I have the satisfaction to state, that such a work is now preparing, under the direction of an editor, whose remarks or corrections cannot but add greatly to its value. Notwithstanding the merit which the separate articles possess, they are not entirely free from the faults incident to whatever is composed for a work already in the The condensation and arrangement, to press. which time is such an essential condition, even with men of the first talents, must be often wanting, in such circumstances; and there are, accordingly, in the articles now referred to, a diffuseness, and
sometimes a want of order, that may easily be corrected, without injuring the authenticity of the work.
Though the Encyclopædia employed Professor Robison very much during the whole of the seven years that it continued, he nevertheless found leisure for some researches of a very different nature. At the period of which I now speak, the French Revolution had arrested the attention, and excited the astonishment of all Europe; and the satisfaction with which the first efforts of a nation to assert its liberties, had been hailed from all quarters, was, by the crimes and excesses which followed, quickly converted into grief and indignation. A body was put in motion sufficient to crush whole nations under its weight; none had the power or the skill to direct its course; what movements it might communicate to other bodies, how far it would go, or in what quarter, it seemed impossible to foretell. The amazement became general; no man was so abstracted from the pursuits of the world, or so insulated by peculiarities of habit and situation, as not to feel the ef fects of this powerful concussion. All fixed their eyes on the extraordinary spectacle which France exhibited; where, if time is to be measured by the succession of events, a year was magnified into an age; and when in a few months one might behold more old institutions destroyed, and more new ones