C. A. Young—Oolor Correction of Achromatic Object Glasses. 455 
same. ‘The spectroscope, arranged as for observation of solar 
prominences, is attached to the telescope, which is first pointed 
at the center of the sun. The portion of the spectrum to be 
examined is then brought to the middle of the field of view, 
and one of the longitudinal dust-lines (caused by particles be- 
tween the jaws of the slit, always far more abundant than one 
would like) is made perfectly distinct by the focussing screw 
of the spectroscope. If the parts of the spectroscope all happen 
to be in exact adjustment, the true dark lines of the spectrum 
and these dust-lines will of course both be perfectly sharp for 
the same focal adjustment; but this is seldom the case for a 
fastidious eye. 
. This adjustment of focus having been accurately made, the 
telescope is then so directed that the edge of the sun’s image 
shall cross the slit perpendicularly, and by sliding the whole 
spectroscope along its supporting bar a position is sought 
where the edge of the solar spectrum shall be perfectly defined 
as seen in the eyepiece of the spectroscope. When this has 
been accomplished the slit will evidently be exactly in the 
focal plane of those rays of the spectrum which occupy the 
field of view. 
Of course if the object-glass is poor, or the air unsteady, no 
point of absolute sharpness can be found, and we have to be 
content with finding a minimum of indistinctness. But with 
ood seeing and an object-glass well corrected for spherical 
aberration, the observation admits a really surprising degree 
of accuracy, successive determinations of the same point seldom 
differing, under such circumstances, as much as half a milli- 
meter. 
Tn this way the differences of focal length for different parts 
of the spectrum may be obtained with satisfactory precision : 
say differences because the absolute focal length is all the time 
changing with the temperature, and that by a considerable 
amount; so that it is necessary to recur to and redetermine the 
zero point continually. This zero in my observations was the 
focal plane of the D lines. cee 
In the table the first column contains the designation of the 
spectrum-lines observed, either by letters, or by their position 
on Kirchoff’s map; the second gives the wave length according 
to Angstrém. The remaining columns explain themselves, ex- 
cept that dF denotes the difference, expressed in thousandths 
of an inch, between the minimum focal length, and that for the 
particular ray of the spectrum in question, while the columns 
headed - contain the same quantity expressed in hundred 
thousandths of the focal length. I have added, for comparison, 
in the sixth column, the results of Lorenzoni for a Fraunhofer 
