Museum March 12th. 1807.
every object which can add to the comforts and conveniences of life are important to us, none more so than that respecting our sight. I know you have improved the frames of Spectacles, and Mr. Mccallister tells me he has sent you a number of Glasses fitted to some of your improved frames—This induces me to write to you on this subject to offer some Idea’s which may have escaped your notice, I have very frequently aided my friends in the choise of Glasses, and trying the focal distance I have often observed that the Image of the Sun with some glasses was given far from the the center of the glass, this I presume has been caused by taking more off one side of the Lens than the other,
query, if a Lens is ground into a perfect sphere, will every part of that Lens give you equal and perfect Vision? the result of this solution determines whether the subject now choosen is worth notice, but an experiment I made yesterday seems to prove that it is very important to take care that the Glasses of a pr. of Spectacles shall accord. To make a pr of Glasses for Mrs. Peale I took a Glass of 16 Inch focus and cut it with a diamond into 2 pieces, and then ground each of them to fit a frame of this shape when I put the glasses into the frames, by accident I put one glass that shewed the Image of the Sun to the upper edge, and the other glass with shewing the Suns Image from the lower edge, When Mrs. Peale put on these Spectacles she observed that her vision was not easey; she thought that the Spectacles made her squint. I then turned one of the Glasses, and the defect was no more complained of. Perhaps it is only by great contrasts that a defect of vision will be sensible, such as the instance with Mrs. Peale’s spectacles: giving the Suns image as above sketched. Therefore I propose to obtain the best vision, will be having the centers of Glasses cut & give the Image of the Sun thus, even for the Spectacles which have different focal distances for each eye, thus: One other observation and I have done. where a person has a like Vision with each Eye, then it is of great importance to have Glasses of the same focal distance, otherwise it makes a blured sight, & I have seen in Stores, Spectacles with a differce of several Inches of focal distance. hence how necessary it is for those who are choosing Glasses to try them by the sun, not only to get a proper focal distance but also to have the Image of the Sun well defined. I have seen Pebbles made in China with so imperfect spherical surface as to be totally unfit for use, although they had an exceeding fine pollished surface. Our friend David Rittenhouse often told me that the most perfect Lens could only be made by grinding them by the hand, that all Machinery for making of Lenses, did the work imperfectly. The veins which is always found in Glass, is a misfortune to those who want large Lens for astronimical purposes, but I suppose for Spectacles, so thin as they are made, is of little consequence, If pebbles in their formation is free from such veins, yet the labour to grind them in the perfect sphere may cause a slight of the workman to make it compleat in grinding before he begins to finish by polishing—therefore, perhaps it is best to be content with Spectacles made of Glass, and being cheap can be frequently changed.
I shall be glad to know your sentiments on the choise of the part of a Lens for use, as I [wish] to be corrected if I have formed a wrong conception [on] the subject. I wish you health, and to continue long in the Station you now fill, if your peace of mind will permit it. I am free to write what I feel, have the goodness to excuse as much as you can, the stile and manners of your friend
C W Peale.
DLC: Papers of Thomas Jefferson.