Imiwovements in the Micro-spectroscope. By F. H. Ward. S27 
prism being at the same time totally reflected. I thonglit the 
matter over for several days, and then conferred with Mr. Hilger, 
than whom I suppose no more competent authority exists on the 
construction of instruments for all the branches of spectrum 
analysis, and the result was a visit to his workshop with some 
prisms to put the matter to the test. A few minutes sufficed to 
remove the right angle from the prisms, and a few touches on the 
polisher gave a brilliant but narrow surface, quite sufficient to indicate 
as a rough experiment whether the method were likely to answer if 
more carefully carried out. Not having a microscope at hand to 
which it could be easily adapted, and wishing to submit the plan 
to as severe a test as possible, we opened widely the slit of a table 
spectroscope and secured the prism in the centre of the opening. 
The battery of this instrument consisted of four large prisms, each 
with an angle of 64°, the faces of which had a width of three inches, 
and as it was so arranged that the ray should traverse them four 
times, the length of the path in the centre of the prisms was about 
25 inches. Placing some sodium salt in a Bunsen flame, we tested 
the result, and found the D lines well separated and beautifully 
distinct and sharp. We also tried lithium, calcium, and carbon, and 
the results were equally good. 
I was so pleased with what I considered the success of the attempt, 
that I directed Mr. Hilger to construct me an instrument for my 
microscope in which three prisms should be substituted for the usual 
slit, and supplied him with a plan by which either one of the three 
could be slipped into position for use. Subsequently, when I had it 
in my possession and tested it by examining the continuous spectrum 
of various solutions, I found the result, however, by no means satis- 
factory : I do not think there is any other word by which to describe it 
than failure. The bands across the spectrum were far worse than those 
from the dirtiest slit ever seen. Trying it again for the sodium 
line, I found it sharp and bright, though only single on account 
of the much diminished magnifying power, and I was reluctantly 
compelled to acknowledge that, good as it might be for bright line 
spectra, it had failed for contiuuous spectra. 
The next question to decide was, whether the failure was due 
to an optical or mechanical cause, whether the theory was right, 
but the practical working out of it imperfect : I was disposed to 
believe it was the latter, and not being deeply versed in optics, I 
could only satisfy my own mind by further experiments. 
I examined with a lens the edges of the linear surfaces, if I 
may use such an expression, of the prisms, and found that they 
were not perfect, that there were minute depressions, tiny splinters 
of glass gone, which, though invisible to the naked eye, were, I 
believed, the cause of the mischief. I had them reground and 
repolished several times with varying results, which I need not 
