142 & P. Langley—New method in Solar Spectrum Analysis. 
respects to Mr. F. Walther, of Philadelphia, the optician, and 
to Mr. William Grunow, the maker, of New York. Not to 
anticipations justified. That it has been possible to me to 
ndertake this research at all at present, is due to Mr. Ruther- 
furd, who has given me choice specimens of his gratings, which 
are so generally known and valued, that it is unnecessary that 
I should describe them. : 
It is desirable that a very clear mental picture should be 
resent to the observer, of the amount of displacement he is to 
expect, for though it is well that he should be ignorant, if possi- 
ble, of the anticipated direction, this knowledge of the amount 
will prevent him at the outset from confusing apparent pene 
tage of a table prepared by Professor Pickering, showing the 
* The following method 
mental picture. In the 3d spectrum of the 17280 line grating the D lines, viewed 
nace : 
a . 
eye at the distance of 10 inches. One of the units of Angstrém’s scale appears 
yi di . The displacement here due 
© fill rather more than #; inch at the 
to rotation is as is remarked rather more than 7; or -08 of a unit, or, referred to 
e of distinct vision, very near of an . It is known that we 
can with the naked eye disti i Ih z#4ya Of an inch or less, in the form of discon- 
i ective 
yet, but can still count 
less than ‘03 units, which again is less than half the amount in question. 
