first power, or 36 inches focal length, for common purposes;i. e. for reading moderately sized prints, and writing in Day-light.

His practice has been, from the period of first using Spectacles, never to employ a higher power than was immediately and absolutely wanted for the particular occasion,— for which purpose he constantly carries with him Two pair of Spectacles; one pair of 36 inches, and another pair of 24 inches focal length; the two pair put on together, serving him for a magnifying power of 12 inches focal length for extraordinary purposes.

The Two pair of Spectacles are for the convenience of being carried in the pocket: at Home, in his study, he has by him all the gradations of 36, 30, 24, 18, and 12 inches focal length, to be used as required.

He observes, that immediately after using Spectacles of a high magnifying power, the Sight does not easily accommodate itself to a lower power; and in such cases, it is necessary to give the Eyes a little time to rest and recover themselves.

This strain of the Eye, and occasion for

Spectacles of a high magnifying power, is particularly found in Mending Pens; when the inconvenience of the Eye not readily restoring itself to the capability of seeing with the ordinary Spectacles is strongly evinced: to avoid the distressing inconvenience, he has a sufficient number of Pens,† to prevent the necessity of mending any of them until he has finished writing. This method of using the higher powers when the lower powers will not be soon after wanted, is always a provident procedure.

There is a convenience in Spectacles of a small power, while they can be used. No object can be distinctly seen through Spectacles which magnify very much, beyond their focal length; and in using the high magnifying power of 12 inches focal length, prints and writings to be re

* Those who find the Mending of Pens rather a difficult Job; I recommend the occasional use of A STEEL - especially when they wish to write very small and

PEN

neatly.

† Mending Pens, and all operations requiring the Sight to be in its best condition, should be attended to early in the Day while the Nerves are brisk, and before the Eyes are fatigued provided the higher magnifying powers are not wanted for this purpose.

ferred to must be placed within that distance of the eye; but with the smaller power of 36 inches focal length such objects may be placed to that greater distance from the Eye for being referred to.*

Therefore the smaller the power, provided it be sufficient, the more pleasant and convenient will be the Spectacles.

By placing upright against a wall, a paper with moderately large printed letters, such as usually occur in the title-pages of Octavo books, he finds the greatest distance he can distinctly see the letters with a good light, to be the Focal length of the Spectacles. For the usual method of measuring the Focal length of Spectacles See Chapter XV.

If any doubt should be entertained of the Two Glasses not being of the same focus, he tries each Glass separately with the same Eye, and the difference is immediately discovered.

Besides the alteration in the convexity of the Eye and the muscular power of contraction

*This supposes a good light, and the object not very minute but such as could be distinguished at the same distance by the best eyes without glasses.

and dilatation, Age reduces the vividness of the illumination on the Retina; in consequence of which, although objects may be sufficiently magnified by Spectacles, yet they may be scarcely visible when the light is not very strong but, on the other hand, a too powerful Light is injurious to the Eye. The Light should therefore be economized for the benefit of the Eye. See Chapter XIII.

The Artificial Light from Candles is rarely too strong; but "the Blaze of Day" is often too intense on white paper, and should be moderated by placing the paper so as to reduce the reflection of the rays of light upon the eye: have enough Light, but no more than is sufficient.

CHAPTER XI.

HINTS TO PERSONS BEGINNING TO WEAR

SPECTACLES,

AND OF READING LAMPS AND CANDLESTICKS.

WHEN persons first put on Spectacles, if they have chosen them ever so wisely-still, they frequently complain that their Eyes feel fatigued, and sometimes even ache, after they

have worn them some time, especially by Candlelight. There is no wonder in this-it would only be wonderful if they did not.

It usually happens, that for some months previous to Persons wearing Spectacles, their Sight has either been strained and weakened by their trying to see what Nature had decreed that they should not,-or, their Eyes have been Idle, and unaccustomed to be used much, especially by Candlelight.

The cause of the Complaint people so often make, that their Eyes very soon tire if they use them at Night, is not, as they seem to suppose, entirely from the inferiority of Artificial Light.

Mr. ARGAND's invention gave us all that we wanted as far as Quantity of Light—and Mr. DEVILLE informs me that the Gas light from the Cocoa Nut Oil has the Quality of Daylight; and that with it, the difference of the colour of Flowers of Sulphur, and that of Wheat Flour, may be easily distinguished, which it is difficult, if not impossible, to see with any other Artificial Light.*

* "The greatest part of Objects, in Candlelight, are always tinged with a Yellow Red, though we are not sensible thereof; because all the objects in view are