"The number of different optical phenomena has become in our time so great that caution must be taken so as to avoid being deceived, and also to refer the phenomena to the simple laws."

“In all my experiments I could, owing to lack of time, pay attention to only those matters which appeared to have a bearing upon practical optics. I could either not touch other questions, or at most not follow them very far. Since the path thus traced in optical experiments seems to promise to lead to interesting results, it is greatly to be desired that skilled investigators should devote attention to it.”

“It will reward enough for me if, by the publication of the present experiment, I have directed the attention of investigators to this subject, which still promises much for physicial optics and appears to open a new field.”

“I wished to find out whether a similar bright line could be seen in the spectrum of sunlight as in the spectrum of lamplight, and I found, with the telescope, instead of this, an almost countless number of strong and feeble vertical lines which, however, were darker than the other parts of the spectrum, some appearing to be almost perfectly black.”

“In order to receive in the eye all the light diffracted through a narrow opening, and to see the phenomena strongly magnified; still more in order to directly measure the inflection of the light, I placed in front of the objective of a theodolite-telescope a screen in which there was a narrow vertical opening which could be made wider or narrower by means of a screw. By means of a heliostat I threw sunlight into a darkened room through a narrow slit so that it fell upon this screen, through whose opening the light was therefore diffracted. I could then observe through the telescope the phenomena produced by the diffraction, magnified, and yet seen with sufficient brightness; and at the same time I could measure the angles of inflection of the light by means of the theodolite.”

“Since the violet rays through the objective of the theodolite telescope have a shorter focal length than the red rays, it is evident why the eye-piece must be displaced in order to see plainly the lines in the different colors.”

“Up to the present time, in experiments on diffraction there has been no instrument, except a magnifying-glass, which could be used with profit; and this may perhaps be one of the reasons why in this field of physical optics we are so backward, and why we know so little of the laws of this modification of light.”