'cone in cone.' I had long tried to explain it, but in vain. One day in arranging some specimens a chance ray of bright light fell on one of them. I observed a peculiar reflection, as if from a multitude of small facets of crystals, lying in nearly the same plane, though in very different parts of the specimen. This fact led me to study the structure from a new point of view, and in a short time all the peculiarities were explained in a perfectly satisfactory manner.

Not to take my illustrations exclusively from one class of objects, I will describe one or two other cases. For many years, in common with all who had studied the subject, I had referred the position of the absorption bands seen in the spectra of various colouring matters to an artificial scale, or in a few instances to the principal Fraunhofer lines. One day, however, whilst rambling over the quiet hills of Derbyshire, it occurred to me, apparently quite accidentally, that such a system had no physical foundation, and that the true method was to express the position of all parts of the spectrum in terms of the wave-lengths of the light at each part. I at once set to work to contrive the means for doing this conveniently, and it was not long before I succeeded. Now that the plan has been once adopted I wonder how it was possible to adopt any other, since it is so simple and self-evidently correct. I am glad to find that everyone who has expressed an opinion fully agrees with me in thinking that for the future this plan is the one that must be generally employed. So far as I

am able to judge, its adoption will almost revolutionise the study of many questions connected with the subject, and cause us to look upon a great number of most important facts from an entirely new and more correct point of view.

It would be easy to multiply such examples and to show the manner in which one train of ideas led to another. Very often the circumstances and train of ideas that have led to a discovery were immediately forgotten in the face of the result, like the scaffolding used in the erection of some stately building; and sometimes the circumstances were connected with the main question in so ridiculous a manner that it would be altogether inappropriate to describe them.

What I have said will, however, I hope, be sufficient to prove the truth of my general proposition, which I may sum up thus.-It is said that in mechanical constructions, in the long run, the dead pressure of an easily moving screw will not counteract the living and ever-acting force of a spring which, as it were, takes advantage of every vibration or change of temperature disturbing the screw, to move it always in one particular direction. In a similar manner, if the mind of an investigator is ready to take advantage of every circumstance that may occur, to press forward his enquiry in the line of truth, the removal of the most formidable difficulties is only a question of time. For my own part, I have generally a tolerably large stock of such questions on hand; and though in some cases I waited

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for years before I arrived at the true solution, yet at length some comparatively trifling circumstance presented itself and the difficulty ceased to exist.

There is also another important consideration intimately related to my subject. It has been said, and I think very justly, that for success in life a man must have a healthy body as well as a soul. I can scarcely believe that what I have been advocating is compatible with such a course of life as leads to decay of health and strength, and with the mind being diverted by the ailments of the body. My own conclusion is that, to ensure this, no small amount of time is necessary for that repose and physical activity which are both essential for health, and which if neglected for the sake of saving time, will in the long run cause the loss of a far larger amount of time by diminished energy and shortened life. I am the unflinching champion of such a well regulated life, and of such a judicious combination of work for the body and work for the mind, as may secure the greatest amount of both physical and intellectual activity for the longest period. One cannot but regret to see that in so many cases too much haste, even in study itself, is bad speed; and into this course many are apt to be led who wish to do more than time will properly allow, and they suffer accordingly.

Perhaps some of my readers may think that many things which I have pointed out as desirable for the successful prosecution of research are very good in theory, but that they could rarely be carried out in

practice. I am quite willing to admit that the physical and mental character of many individuals would scarcely allow of their carrying out such a plan as 1 have sketched, but at the same time I am none the less convinced that it is a possible standard, and certainly the thing to aim at for the progress of discovery. Even those who might deny the possibility of such a kind of life will, I think, admit that if it were possible it would do much to advance scientific knowledge. One thing, however, is clear, and cannot be denied; no one can even remotely approach to this mode of life and continuous observation whose mind is constantly engrossed with other cares-whose thoughts are necessarily directed to the consideration of how he can provide for the needs of each coming day, or how he can extricate himself from or avoid pecuniary embarrassment. Whatever the experience of others may lead them to think, mine has been amply sufficient to convince me that I never could have done what I have been able to do, if it had been necessary for me to attend to any business or profession as a means of support. Though I wish I had been able to do more, yet if I had been interrupted by the cares of practical life I should certainly have done far less, and in all probability the general quality of the work would have deteriorated to a still greater extent, since the opportunity and inclination to attack wide branches of enquiry would have been greatly reduced. I am very far from being one of those who think that everyone should do the same

thing, or that precisely the same conditions are the best for all. I give the results of my own experience, and must at the same time say that I think that they apply with more or less truth to the majority of

cases.

It may, however, be said that though attention to an ordinary business or profession may thus interfere with the prosecution of original research in any particular branch of science, yet the duties of a professor of that same branch might not only not interfere, but be a positive advantage. It has indeed been urged that the mind and character are steadied by a routine employment, and that this steadiness acts upon the quality of the scientific work done in the residual time. I will not deny the possibility of this in some cases. Some individuals may be met with who are so unbusinesslike and unmethodical in their actions that it may be desirable for them to wear constantly some such sort of a strait jacket to keep them within anything like reasonable bounds, just as we may find some who will never do right when it is possible for them to do wrong. It would, nevertheless, I think, be very unwarrantable to conclude that any large proportion of those who are able and willing to devote themselves to a truly scientific career are characterised by such a moral weakness. If the conscientious study of natural science is not sufficient to give a man enough ballast and fly-wheel to insure steady and continuous activity, I should look upon him as being thereby unsuited for