^jTurali, dIH'^^^^^^^^ ^oyal Microscopical Society. 291 
" If parallel rays * from S fall on the grating Gr, at any inclination, 
the eye at E (perpendicular to the grating) will see a colour pro- 
duced by the interference of the rays whose paths as reflected 
from the bars to E differ by half an undulation from the colour 
complementary. But this colour will not be seen unless the eye is 
at E. For other directions as F and F' the interference cannot 
take place. 
"The question is not, Avill these bars be coloured, but will' 
they be visible. Nobert argues that when there is no colour, the 
complement to no colour, i. e, the whole light, must be suppressed. 
That is all I have ever been able to make of his argument or 
Fraunhofer's. This is not only theoretically not proved, but ex- 
perimentally not true. It would be true both experimentally and 
theoretically in light positively monochromatic, provided the eye 
received only the perpendicular rays at E. But with an objective 
that takes in a cone of an angle of from 140° to 175^, it is nonsense 
to talk of this question as one settled by theory. 
"We shall continue to see closer lines just in proportion as 
microscopes and modes of illumination are improved. Probably 
there is some physiological difference between individuals. All 
these images are faint, and keen eyes will see them better than 
dull ones. It would be a good test of the truth of Nobert's hypo- 
thesis to try, if, with a pure monochromatic red or yellow light, 
the thirteenth band of the nineteen-band plate is resolvable. 
" On reviewing the table of wave lengths, and comparing vdth 
Nobert's statements as to the rulings of the nineteen-band plate, 
I am ready to affirm that, if his theory is true, not even the ninth 
band can be resolved in monochromatic yellow light." 
At the time I received this letter from President Barnard, I 
had already resolved the sixteenth, seventeenth, and eighteenth bands 
with a new immersion t eth, constructed for the Museum by Messrs. 
Powell and Lealand, of London, and subsequently I succeeded in 
resolving the nineteenth band with the same objective. With this 
lens a series of photographs of these bands were then prepared by 
Dr. Curtis. These accompany this paper, and will be presently 
described. 
A careful count of the lines in each band gave the following 
results : — 
ISth. band ,. 45 lines 
16th „ ..48 „ 
17th „ ..51 „ 
18th band .. 54 lines 
19th „ ..57 „ 
In obtaining the a,bove results I illuminated the microscope, as 
in my former work on the Nobert's plate, with a pencil of mono- 
* Colonel Woodward has sent no drawings for a diagram illustrative of these 
remarks, but as the meaning is tolerably plain to students of optics and the remarks 
are of import, we do not wish to excise the paragraphs. — Ed, M. M. J. 
X 2 
