54 ALEXANDER JOHNSON ON NEWTON'S OPTICS. 



consequence, deprive him of the discovery of the dark lines. It was not, however, an 

 inevitable consequence, as may be seen by making the experiment with a crown glass 

 lens alone, as he did. 



Experiment ivilh a crown glass lens. — For this purpose I separated the parts of one of 

 the object glasses that I had used previously, and tried further what could be done with 

 the crown glass convex lens thus obtained. Its focal length was eighteen inches and 

 aperture two and three-quarter inches. The results are as follows : 



With the slit of one-half mm. width I counted on one occasion ten dark lines, and on 

 others eight. 



Widening the slit to one-twentieth of an inch (a width mentioned by Newton), I saw 

 four lines, viz , E, b, F and G, of Fraunhofer. I noted at the time that it wan quite impossible 

 to overlook, them. 



Opening the slit still farther to the undesl extent recorded by Newton, viz., one-tenth of an 

 inch, tivo lines (F and Gr) ivere still visible, and impossible to be overlooked. There were also 

 traces of others. This experiment was repeated more than once, with the same result. 



Round hole — I tried also a round hole. When the diameter was one-twentieth of an 

 inch I still could see one line (G), but with a width of one-tenth inch could see none. 



The following is a summary of the conclusions arrived at : 



1. That if Newton had had an achromatic lens, his method was so effective that it 

 would have been impossible for the dark lines in the spectrum to have escaped his notice 

 whether he used a slit or even a round hole one-tenth of an inch in diameter, without 

 taking into account the slit one-twentieth of an inch " and narrower " 



2. That even with a crown glass lens the lines must have been seen had he been 

 ordinarily fortunate in the particular lens used. 



3. That the rise of the error concerning the slit seems to have been contemporaneous 

 with the introduction of spectrum analysis, judging from the dates given above. 



