See ae ee ee ee ee Te eb ea, AE SO, Ree eae Oy ke SS Pee a 
Chemistry and Physics. 165 
here is no evidence), have been made with glass of different 
thicknesses. When projected in the lantern, this variation in 
thickness would necessitate a change in focus and so cause a chan nge 
in the magnifying apie The smaller sizes of 2S otographic 
glass vary in thickness from one to two millim eters. Conse- 
a 
cause of inaccuracy must here be pointed out. From the data 
given by the author, it may petra be calculated that his original 
photographs of the ox gen spectrum, taken with two prisms of 
60° and with lenses of ten inches focus, could not have oe over 
gion from G to H. Since Angstrém’s 
sheen as thrown on the screen must have been magni ed one 
- : : 
absence of any precise statement the reader has to make the cal- 
culation for himself; but the figures above given cannot be far 
astra 
ake there is only an appearance of accuracy when the 
attempt is "made to fix the position of the oxygen spectrum lines to 
hundredths of one of Angstrom’ s scale divisions. The projection 
method by which the solar lines were measured, has already been 
Moreover, only forty-seven iron lines were used in all, or one to 
every eleven sail divisions; the reading being to one one-hun- 
dredth of a scale division, or 1,100 numbers one iron line 
Since the author ed no wave-lengths directly, he was 
obliged to itor Pont a considers “ portion of the curve from the 
wave-lengths o and air lines already given by various 
authorities.” e atin were taken, page 258, from Watts’ 
ments employed in the paper before us. 
