390 Notes. 



reached. If continued further, a new line makes its appearance on 

 the one side just as the line on the other is going to disappear. 

 Now when we have a grating on the stage, it can be shown that in 

 the image the diffraction patterns, due to neighbouring lines in the 

 object, overlap, and interference occurs in such a way that as any 

 image line loses in brightness from one object line, it gains in 

 brightness in just the same proportion from the neighbouring object 

 line. Thus the brightness as seen remains constant, and so far as 

 appearance goes it is the same line which travels right across the 

 field. 



My intention this evening was only to bring some visual 

 evidence in connection with the interesting matter which Prof. 

 Everett has brought to our notice. I fear I have been explaining 

 my experiment at too great length ; if so, my excuse is that I was 

 desirous to bring out not only the points of similarity, but also the 

 points of dissimilarity which may result from creating difference 

 of phase amongst the spectra by different means. 



"When the above note was written, I thought the experiment 

 was an entirely new one, but I have since found that a somewhat 

 similar one was made by Abbe, and referred to in a catalogue of 

 optical measuring instruments issued by the firm of Carl Zeiss in 

 1893. 



In Abbe's experiment the chief maximum is blocked out, and a 

 phase difference created between the two maxima of the first order, 

 and it is pointed out that, under these circumstances, the number 

 of lines in the image is doubled, and that the strife wander. 



An Attachment for reading the lines in a direct-vision SiKctroscope 

 By E. B. Stringer, B.A., F.E.M.S. 



This attachment (fig. 63) may, perhaps, be of interest to the Society, 

 since an arrangement of the same kind might easily be made for 

 a micro-spectroscope. It affords a more accurate means of reading 

 the lines than the reflected scale which is generally used, whilst it 

 is even more convenient, as the scale does not require independent 

 illumination. 



It consists of a light and rigid arc of phosphor bronze, of about 

 40 degrees, and 6| in. radius, cast in one piece with the broad ring 

 by which it is firmly clamped to the body of the instrument. The 

 arc carries a millimetre scale, which is divided in white upon a 



