Thus for an Object-glass of 1 Inch and a half,- or fifteen 10ths in Diameter,―ifa Magnifying power of 4 times be desired, divide 15,the number of Tenths the Object-glass is in diameter by 4, the Magnifying power: 4)15(ths and ths, the diameter of the 12 3 I would have the Eye-glass somewhat larger than the Pencil of Rays, i. e. for an Object Glass of 14 inch in diameter, and magnifying 4 times, I think that my Eye sees easiest, when the opening of the Aperture in the Eyehead is about gths of an Inch in diameter. The Eyehead should be of Black Ivory, not less than an inch and ths in diameter, and made concave-something in the form of an Eye-bath or a shade on the side similar to those prefixed to the Tubes used for viewing Pictures so that it may form a Screen around the Eye, and prevent the intrusion of any rays upon the retina, except those coming directly from the Opera Glass- this, will not only improve the Vision very much, but also render it much easier to the Eye. (See page 86 of this Vol.) N The average Distance, at which a Common Eye, can see distinctly, the expression of the Human Countenance (in a good light) has been calculated to be about 15 feet. The average Distance, between the Actor and the Spectator, in the Boxes of a Theatre, is about 4 times 15 feet, i. e. about 60 feettherefore, to shew distinctly, - an Opera Glass must magnify 4 times. The Bell Operas, which have only one sliding tube, are the best; in those which have more tubes, the centre of the Objectglass and the centre of the Eye-glass are very seldom exactly opposite to each otherand in proportion as they are Eccentric, Vision is Imperfect, and the Instrument (according to a very usual and very useful Optical phrase) is said, to be-Out of Adjustment. Look with a scrutinizing Eye through Operas which are elaborately ornamented and have many Tubes for it is one of those General Rules which has the fewest exceptions, that those which are so very pretty to look at, are not seldom mere Toys, which are made to be looked at, rather than to be looked through. This Caution is quite needful, friendly Reader for I think I have been as much puzzled to produce arguments to persuade my friends that Opera Glasses are not always to be chosen for their Tubes, as I have had to assure them that a Piano-Forte will not always make good its claims, to favour from the Ear exactly in the proportion that its external appearance happens to please the Eye. As Ten are made of the plain mountings, to One of the other, it is, in like proportion, easy to pick out a fine one. The majority of the Opera Glasses which are sold at Trinket and Toy-shops, magnify so little, and are of so little use that many people who have good Eyes, say with truth, that they can see as well with their naked Eye - I have met with many who have said so to me but, when I have shewed them a good Opera Glass, magnifying 4 times, precisely tuned to the peculiar pitch of their Visual Organ-they have All, acknowledged, with astonishment and delight, the surprising aid that their Sight received from the Eye-in vigorating power of Optics, which enabled them to See Persons in the most distant parts of the Theatre, almost as distinctly, as those who were within 15 feet. 1 After a deliberate, and fair trial of the Magnifying powers of 3—4—5—6, &c., my Verdict is, that for use in the Theatre, for Common Eyes, a Magnifying power of about 4 times, is decidedly, the most generally useful and agreeable,—and what is of great importance, as the Eye is sometimes before it for a long time, it is much easier to the Eye than a Higher Power. Persons who are extremely Short-sighted, (See the Note at the foot of page 103), may find a Concave which produces a power of 5 to a common Eye, not too much for them. It is desirable, that the Magnifying Power be as Low as can be, that the Field of View may be as Large as can be; because the latter depends partly on the former moreover, the vapour from the breath of a large Assembly, and the Smoke from the numerous Lamps, &c. prevent our using much Magnifying power. When I suggested my opinion of what ought to be the Magnifying power of an Opera Glass to an Optician-I was told, "It has been tried but the less they magnify, the more people like them; and indeed, those seem to me to be most approved, which magnify so little that they scarcely require any Adjustment." Exquisite Opera Glasses, that have no focus!-and are equally distinct, whether all the Eye-tube is pulled out, or all put in!!or you put up the Large, or the Small end to your Eye!!!-My Optical friend added, with a smile, "You may laugh, Sir, but every body has not time to listen to a Long Story aboutFOCUS,-ADJUSTMENT, &c. &c. &c.-Gentlefolks don't like a troublesome thing,that requires Half a Minute to set it in some particular form, before they can see through it." OPERA GLASSES have been one of my favourite hobbies for the last Thirty Yearsand to gain the information contained in these pages, I have carried my experiments to the greatest extent possible, for I think I have |