Professor Trowbridge delivered the first of the Jefferson laboratory course of lectures last evening, on the subject of Spectrum Analysis. There was a very large audience present of which a large proportion were college men.

The lecturer first called attention to the two simple propositions of light namely: Light moves on straight lines and light is refracted on passing between two mediums of different densities.

Sir Isaac Newton was the first to observe the effect produced by a prism on light but was opposed in his theories by Goethe.

Light has energy; is, in fact, a form of energy. After Sir Isaac Newton's discovery the subject was not touched for some time. When it was taken up again the spectroscope was invented.

We have means of ascertaining the waves of energy coming from a nail which is but slightly heated. The two carbons of an electric light were then projected on screen and the light coming from the incandescent carbons shown.

By the old method the spectra of various metals were obtained by burning the metal in the flame of a Bunser lamp placed before the slit of a spectroscope which had one prism. As a consequence the lines of the spectra were crowded together and confused. The modern method of observing spectra is by taking photographs in the spectroscope and composing the result with a photograph of the Sun's spectrum.

We can extend the spectrum beyond the visible part. The great interest of to-day, however, centers in the invisible rays of heat. Until very recently, photographs could only be taken by violet rays of the spectrum. Now, however, the method has been extended so as to include the yellow and red rays, known as the all-day exposure method. Here the lecturer showed a photograph taken by Mr. Burbank, '89, of two sodium lines. It is the first one of the kind that has ever been produced. America has done more in the last fifty years to bring photography to perfection than any other country, although much has been accomplished by recent scientific investigation in England and France. The old spectra of stars were mere dots, but now Professor Pickering has been able to take photographic spectra of them as large as the old spectra of the sun.