Hand-Book for Opticians; A Treatise on the Optical Trade, and Its Mechanical Manipulations (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1895 edition. Excerpt: ...to put the axis of the cylinder ninety degrees from it. Sometimes the faulty meridian of one eye is at right angle to that of the other eye; when in binocular vision they will correct each other; it is, therefore, absolutely necessary to test each eye separately. This kind of astigmatism is called simple le_1/pemnetropic or myopic astigmatism, according to the nature of the correctingr cylinder, which will be either plano-convex or plannconcave. If the astigmatism be hypermetropic, the axis of correction will be the same for both far and near, but if the astigmatism be myopic, the axis of the correcting cylinder should be for near vision at right angle to the one required for the distance. Vhen presbyopia is combined with myopic astigmatism, and requires for distant vision a concave cylinder axis 90, then the glass for reading should be a convex cylinder axis 180, thus correcting only the meridian that is not myopic. For near vision the myopic meridian either requires no glass at all, or one that is weaker, because its near point is of a shorter distance than the emmetropic meridian. The second variety of astigmatism is the combination of astigmatism with hypermetropia or myopia. Its correction depends entirely upon the relative proportion of each deficiency. Prof. Airy, for instance, had to correct his astigmatism before he could equalize the focal distance of his eyes by the addition of concave spherical lenses. But if hypermetropia or myopia is in excess of the astigmatism, we better correct them first in the usual way, and finish off by adding the correcting cylinder. It happens sometimes, after we have evidently corrected the full amount of hypermetropia or myopia, and added the cylinder, with axis at right angle to the direction of...