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lY. — The Present Limits of Vision. Further Elucidations. 
By Dr. Eoyston-Pigott, F.K.S., &c. 
{Taken as read before the Eoyal Miceoscopical Society, October 4, 1876.) 
It has occurred to me tliat perhaps several of the readers who are 
UD acquainted with my methods may find a few explanations not 
devoid of interest, especially those who wish to repeat the experi- 
ments for themselves. 
I therefore now propose to say a few words : 
(1) On my method of producing vivid miniature disks of the sun. 
(2) On measuring them theoretically. 
(3) On the application of the undulatory theory of optics to 
the calculation of the diameters of the enlarged spurious disk, and 
the breadth and size of the primary black ring and of the successive 
bright and dark rings of the diffraction. 
1. Vivid Solar Miniature Bishs^ and hoiv to get them. 
Arrange an Amici prism in the sunshine so that by total re- 
flexion you can see an aerial brilliant image of the sun ; mine has 
an imbedded lens, giving a focal length of three inches. In front of 
this I place between the prism and the eye a \ objective for the 
telescope ; but for the microscope I place the prism 100 inches 
distant, and fix a fine ^th as condenser, to form the miniature in 
the focal plane of the microscope, without the use of any silvered 
mirror, which utterly spoils these charming phenomena. Viewing it 
with a fine ^th and a G eye-piece, by lengthening the tube the over- 
powering effulgence may be subdued so as to render inspection 
tolerable ; but no blue shade or coloured glass must be employed. 
I am glad to be able to state that a power of 1000 subdues 
the sunlight sufficiently without coloured glass ; but a little 
care is required in selecting the eye lenses, so that the field of 
view may just embrace the whole spectacle. To those who have 
never seen these glorious displays, I may strongly recommend this 
interesting department of microscopical optics, as at once delightful 
and recondite, yet most instructive. 
The telescope, with a power of 300 diameters at a distance of 
100 feet, gives, up to a certain point, almost identical phenomena 
for axial displacements. You are presented with the same richly 
enamelled patterns of overpowering brilhanC'e and exquisite forms. 
Whether, therefore, the telescope or the microscope be used, the 
same results follow within the same limits of aperture. 
Now for the question of destruction of diffraction. This term 
is used to denote the effect of undulations passing through apertures 
or gratings. Interferences result which are more or less con- 
spicuous according to the intensity of light. But ivith mild or 
diffused light they are almost totally invisible. 
