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through their Opera Glass, keep ever and anon, pawing, and wiping the Eye or the Object Glass; neither of these should be touched,except when it is absolutely necessary to clean them, and then, only with a bit of soft Leather, fine Linen, or the finest Silver Paper.
The Sliding Tube soon becomes dirtied by the dampness of the hands;-to avoid this, do not touch the Sliding Tube, but take hold only of the Neck of the Eye-head, and adjust by that. The Sliding tube must be wiped occasionally, and at the same time wipe round the cloth lining of the larger tube-so that it may slide smoothly if it will move only by fits and starts, you will not be able to adjust it accurately.
I have heard persons (unacquainted with the Laws of Optics,) complain, that an Opera Glass magnifying 4 times, has not so large a Field of View as a Glass which magnifies only 2this cannot be remedied; their only alternative, is to have a small Field distinct, or a large Field of little or no use- with a power of 4 they have in quality, what with 2 they have in quantity, the objects which they do see, they see much more than twice as distinctly.
In 1801 there sprang up" the Grand Dandy Opera Glass," with a Great Eye glass, as big, as its Object glass! - it was imagined, — by the Amateur Optician who introduced it, that by increasing the diameter of the Eye-glass, the field of view was proportionately increased,— and that the Stops* which had heretofore been placed as Sentinels to prevent the intrusion of false light, were impertinent impediments.
The fact is, these great Grand Dandy Eye
*These are often opened too Large, especially in Opera Glasses made with a Single Object Glass.· As the Eyeglass should be a little Larger-so the Stop should be a little (very little) not more than th of an Inch less, than the cone of Rays coming from the Object-glass.
The effect of the Stop varies according to its distance from the Object-glass.
The Diameter of the Aperture of the Stop, must be proportioned to the Degree of the Magnifying Power-the former must be contracted, in proportion that the latter is increased if it is too large the Vision will be confused and indistinct by the intrusion of False Light, if it is too small part of the pencil of rays will be cut off.
The subject of STOPS is excellently illuminated in a Paper on "Indistinctness of Vision," caused by the presence of False Light in Optical Instruments; and on its Remedies, by C. R. GORING, M.D., in the Journal of Science, No. xxxiii. for April 1824.
glasses, actually magnified very little, and on that account had a very large Field-hold a Card with an aperture of half an inch in diameter, before the large Eye-glass-you will find the field of View as large, and the Vision as bright through that, as through the Eye-glass of an inch and a half in diameter:-or, apply a Concave of an inch and a half in diameter, which makes them magnify 4 times, and you will find that the field of view is not a hair'sbreadth wider through that than it is through an Eye-glass of only half an Inch in diameter.
Vision through the Smaller, is more distinct than through the Larger Eye-glass: Eye is apt to wander about before large Eyeglasses, and the margin of the field of view is curved, Vision is perfectly distinct, only, when you look precisely through the centre of the Eye-glass.
I shall relate some further Observations, PRO AND CON Concave Eye-glasses when I treat on the comparative Illuminating Powers of CASSEGRANIAN, GREGORIAN, NEWTONIAN,
* See several Remarks on Concaves in Sir W. Herschel's paper on the Quintuple Belt of the planet Saturn. Phil. Trans. for 1794, vol. lxxxiv. p. 28; and in vol. cv. p. 296.
and ACHROMATIC TELESCOPES in the Second Part of this Work.
The field of view in Telescopes constructed with Convex Eye-glasses, -is regulated, by the Stop which is placed in the focus of the 1st Eye-glass, or that next to the Eye; - the diameter of the Stop is regulated by the diameter of the 2d Eye-glass, the diameter of which, varies according to the Magnifying power used. If the Stop be opened larger than the 2d E. G. it will produce a strong Orange Colour around a very indistinct margin—with the same Magnifying power, the field of view is the same, whether the Aperture of the Telescope be One Inch, or Three. This is easily proved, by contracting the Aperture of a Three Inch Telescope to One Inch, when the ⚫ field of view will remain the same;you will find that the only difference in its appearance, is the diminution of the brightness of it.
But with Perspectives or Galilean Telescopes, or OPERA GLASSES which have a Concave Eyeglass The Field of View, when they do not magnify more than Twice, depends in a great measure on the Diameter of the Object Glass.
If an Object Glass of an Inch and a half in
diameter, is made to magnify only twice, it will have a much larger field than an Object Glass of an Inch in diameter.
But when a Magnifying power of 4 times is applied, - an Object Glass of an Inch aperture, will have very nearly as large a field as one of an Inch and a half- but through the larger aperture, Objects will appear twice as bright, as they do through the smaller, the Illuminating power of the larger, to the smaller aperture, being as 22 to 10.-These facts any body may prove, by looking through an aperture of an Inch and a half, and then contracting it to an Inch.
Some of our most valuable discoveries have been purely accidental, as little the result of scientific investigation as the Telescope, which was found out by Children playing with the lenses in a Spectacle-maker's workshop: these Great Grand Dandy Eye-glasses, as big as Object Glasses, led me to consider-what Diameter of Eye-glass and Object Glass is actually useful.
Theoretical Opticians, have said, that if the Eye-glass, be as large as the Pupil of the