and work in the impetus they have given to younger men, who hung about them when living, and fill their places when dead. This feeling is forced on me very powerfully by one fact which it was my duty to state in my former address. It was this. I said "The annual addition to our Transactions this year (1857-8) contains but one paper. That paper is by Mr Stewart, and is of unquestionable merit. I have great pleasure in learning that Mr Stewart is continuing his researches on radiant heat, a branch of experimental science which owes so much to members of this Society, and the papers on which alone suffice to stamp our Transactions with lasting value." I need not say that the papers to which I referred were, for the most part, contributed by one member of the Society, the man who had for the previous twenty years of my connection with it most conspicuously illustrated in his own person the position which he himself lays down, that the vitality of a Society like this is kept up by a few ardent workers, whose contributions like the life blood trickle through the veins of the Society, and warm and animate its remotest members. Forbes was a man pre-eminently constituted to be a leader in a Society like this-cold, yet thoroughly affable; always at work, yet always accessible; keen and resolute in maintaining his own claims, yet ever open to the claims of those about him; and above all, straightforward even at the risk of treading on the toes of men with whom he might happen to come in contact. Forbes was one of two members of the Society whom I had the honour of knowing before I came to Edinburgh. He was introduced to be by Dr Whewell, and was ever my fast friend. Though perhaps a little out of place in such an address, I cannot resist the inclination, in holding up his ardent, hopeful character to the younger members of the Society, to read a portion of a letter which I received from him on his last return from the Continent to lie down and die. It is written in bed and in pencil. He says

"I was very glad to hear from you. You will not expect a long reply. I am, in fact, very weak, though I made out the journey wonderfully, by the help of every luxury and indulgence which modern railways and hotels afford. Here on English ground I am content for the present to rest and be thankful; to leave the issue to a merciful Providence, in whose goodness and guidance I place my full reliance. Of course, I do not look at present to any further movement

It is most gratifying to me to know that you and other old friends still thought of me as a successor to Sir David Brewster [in the Principalship of the University]. That, of course, I at once surrendered. It was the last remaining rag of worldly ambition which remained to me, and I surrendered it cheerfully."

I have selected Forbes out of the many who are called up to memory by a reference to what I said in this place twenty years ago, both because he is most vividly associated in my mind with this room and this Society, and because he is about the very best type I could select of a man who derives benefit from the associations connected with a Society like this, and who in his turn reflects those benefits most powerfully on others. The solitary paper which I have mentioned as the sole product in our Transactions of the Session, was an early product of a mind lit up by a spark from Forbes's anvil. Balfour Stewart had been a worker in Forbes's workshop, and had imbibed much of the spirit of his master. He is now one of our Honorary Members; a fact which sufficiently expresses the opinion of this Society of the manner in which he has been doing his work. It would not become me to pursue the subject of the influence of one mind upon another, due to their close contact, by singling out some of the fervent workers in this Society as the insensible creations of the good men who have lived before them. The fact is patent. Good men have raised up good men to succeed them. Our Transactions of the present period contain papers not a few destined to take their place in the permanent repertories of science. We have about us workers whose praise is wide spread, but this is not the place to sound it. The only word I can venture on as both encouraging for the present and hopeful for the future, is the remarkable number of young men who are just entering on their work. In the fasciculus of the Society's Proceedings just issued, I count not less than eleven names of young men just entering on their career of investigation. How many of them have caught their inspiration from contact with those older workers who have been long among us! How many have been drawn out and cheered on by the associations of this room!

I. On the Action of Light on the Iris. By William Ackroyd, F.I.C., &c. Communicated by Professor M'Kendrick.

Sect. I. It is well known that the movements of the iris are due to the stimulus of light, but I am not aware that any experiments have been hitherto made to determine the approximate quantity of that agent necessary to bring about this involuntary action. The usual way of observation precludes refined experimenting, it being customary to watch the iris of another person or animal whilst under the influence of varying amounts of light, or one's own iris by means of a mirror. Three methods will be described here, and I believe that one at least may afford a means of getting new data on this and other points.

Sect. II. The first and second methods depend upon the following facts:-That, if a divergent bundle of rays emanate from a small surface or hole, very near to the eye (say about 30 mm. off), this surface or hole is the apex of a cone of light whose base is the pupil; that every movement of the iris affects the area of this base, which VOL. X.

appears as a circular luminous field; and finally, that I find these alterations of area, so easily seen, may be taken as indications of the movements of the iris.

The third method is equally simple. The lachrymal fluid on the surface of the cornea affects the image of any light source, such as a lamp or star, and by refraction causes the appearance of rays to emanate therefrom.

It is obvious that the length of these rays must be regulated by the iris, this organ being nearer to the retina, hence when the pupil contracts the rays ought to shorten, and when the pupil expands the rays ought to lengthen out. Such I find to be the case.

Sect. III. The First or Reflection Methol.-The following is the simplest form of the experiment I have been able to devise. Burnish the head of an ordinary brass pin, and place the pin up to head in a black hat. Now, with one eye shut and your back to the light, bring this pin-head near to the other eye so that the light may be reflected into it from the convex surface of the pin-head.

One sees a circular luminous field, with projecting hairs at the bottom which belong to the top eyelid.* Globules of the lachrymal fluid also appear at each wink.

Expt. 1. Shade the light from the observing eye for a few seconds, then let the light fall on it again. Notice the alteration in area of the field of view. The field contracts, then expands slightly, and oscillates until the iris is adjusted for the amount of light falling into the eye.

Expt. 2. Observe the pin-head with the right eye for some moments, the left eye being closed. Open the left eye. The iris of the right eye is seen to move markedly, the pupil contracting. Here the iris of the right eye is moved by the light entering the left

one.

Expt. 3. With everything as in Expt. 2, have both eyes closed and only open the right or observing eye. There is contraction of the pupil, but apparently no more marked than in Expt. 2.

A simple method is here suggested for demonstrating to one's self the inverting action of the crystalline lens. With everything as here described, take a needle and bring it across the field of view close to the eyelids. If it more downwards, it appears to move upwards; if it be moved upwards, it appears to come downwards.