It would lead us far beyond the object of these lectures to dwell upon the numerous interesting and important results obtained by Secchi, Respighi, Young, and other distinguished men who have worked at the chemistry of the sun and its appendages. Nor can I do more at present than make a passing reference to the excellent labours of Dr. Huggins in connexion with the fixed stars, nebulae, and comets. They, more than any others, illustrate the literal truth of the statement, that the establishment of spectrum analysis, and the explanation of Fraunhofer's lines, carried with them an immeasurable extension of the chemist's range. The truly powerful experiments of Professor Dewar are daily adding to our knowledge, while the refined researches of Capt. Abney and others are opening new fields of inquiry. But my object here is to make principles plain, rather than to follow out the details of their illustration. SUMMARY AND CONCLUSION. My desire in these lectures has been to show you, with as little breach of continuity as possible, something of the past growth and present aspect of a department of science, in which have laboured some of the greatest intellects the world has ever seen. I have sought to confer upon each experiment a distinct intellectual value, for experiments ought to be the representatives and expositors of thought--a language addressed to the eye as spoken words are to the ear. In association with its context, nothing is more impressive or instructive than a fit experiment; but, apart from its context, it rather suits the conjurer's purpose of surprise, than the purpose of education which ought to be the ruling motive of the scientific man. And now a brief summary of our work will not be out of place. Our present mastery over the laws and phenomena of light has its origin in the desire of man to _know_. We have seen the ancients busy with this problem, but, like a child who uses his arms aimlessly, for want of the necessary muscular training, so these early men speculated vaguely and confusedly regarding natural phenomena, not having had the discipline needed to give clearness to their insight, and firmness to their grasp of principles. They assured themselves of the rectilineal propagation of light, and that the angle of incidence was equal to the angle of reflection. For more than a thousand years--I might say, indeed, for more than fifteen hundred years--the scientific intellect appears as if smitten with paralysis, the fact being that, during this time, the mental force, which might have run in the direction of science, was diverted into other directions.