A BETTER KNOWLEDGE OF OPTICS. 205 
action of a diamond in ruling lines upon glass.”—(American 
Society of Microscopists.) In this paper it is remarked that since 
the death of Nobert, Mr. Fasoldt, of Albany, stands easily first in 
the fine art of ruling. Prof. Rogers has recently taken up this 
subject with the view of testing the claim of Mr. Fasoldt, that he 
has succeeded in ruling lines one million to the inch, and especially 
by the claim that the existence of a spectrum in the bands is an 
evidence of the reality of the separate lines. We cannot learn 
that any one has yet succeeded in photographing a Fasoldt plate 
as high even as 100,000 to the inch. Photography, the author 
remarks, offers the evidence, somewhat negative in its character, that 
the limit of visibility, as distinguished from resolution, is reached 
with lines having a width of about syq/5g9 th of an inch, and lines 
of this width are the finest that have ever been photographed. An 
excellent example of Prof. Rogers’ own gratings was presented this 
year to the Royal Astronomical Society, and one of his glass 
centimetres, containing 1001 lines to the centimetre, was presented 
to the Royal Microscopical Society. Those who are practically 
interested in diffraction rulings, will find much in Prof. Rogers’ 
paper worthy of careful consideration ; for it would almost appear 
that the microscope has, in this matter, reached its highest visual 
possibilities, little or no progress having been made (certainly none 
whatever in this country) since the resolution of Nobert’s nine- 
teenth band. 
A BETTER, KNOWLEDGE, OF OPTIGS. 
HIS is perhaps the most important of all means or verification 
of microscopic observations. Without this all the rest will be 
in vain. The most elaborate or the simplest apparatus will yield 
no real gain of knowledge to the world unless the eye be trained 
to comprehend what it sees, to interpret the appearances that 
present themselves and discriminate the causes that produce them, 
and so trace back the effects of the lenses themselves, of the dia- 
phragm, of the obliquity of the light, and the effects due to the 
real structure of the object under examination. The mathematic 
reasonings of Helmholtz and still more those of Abbe into the true 
theory of microscopic vision may not need to be followed by every- 
one who would use the instrument, but to be acquainted with the 
main facts of Abbe’s theory—to comprehend the doctrines he has 
propounded and the experiments by which he has made it plain, 
so as to use it in the interpretation of what the lens reveals is as 
necessary for him who would be a well-skilled observer as for him 
who would improve the powers of the instrument itselfi—American 
Society of Microscopists. 
