Ocular refraction and the shadow test . chromatic aberration, the images fonned by strong sphericallenses are tinged with these colors. In figure 58 the spectrum color band is shown as having sevencolors, but it is merely a sub division of the blue into two shades,cyan blue an : ultramarine blue, according to a later idea of thespectrum. 54 OCULAR R L F R A C T I O N. Having found that various powers may be obtained in lenses,some method of numbering or indicating their power must necessarilybe devised. Almost all knowledge is obtained by comparisons withsomething that we can easily comprehend
Ocular refraction and the shadow test . chromatic aberration, the images fonned by strong sphericallenses are tinged with these colors. In figure 58 the spectrum color band is shown as having sevencolors, but it is merely a sub division of the blue into two shades,cyan blue an : ultramarine blue, according to a later idea of thespectrum. 54 OCULAR R L F R A C T I O N. Having found that various powers may be obtained in lenses,some method of numbering or indicating their power must necessarilybe devised. Almost all knowledge is obtained by comparisons withsomething that we can easily comprehend, thus we form our estimatesof height, distance, value, etc. by adopting some definite ////// withwhich to calculate multiples or sub divisions. As the most convenientproperty of a lens upon which to base a system of numbering is tomeasure its length of focus it was natural to note this in inches, theinch being the linear unit. A /ens Iiaving a focal length of one inchwas therefore adopted as the unit, this really established a uniform. Disper: by a pri: ed bv Figure 58. Band of light passing through slit .A, tP, forming spectrum band S. system of numbering which was a great improvement over oldermethods, by which each lens grinder distinguished certain lenses bya name, or a number that represented a totally different value fromthat of another maker. While an improvement, the inch system left much to be desired. Of a one inch lens it may safely be said,it isnever used in the correction of an error of refraction, those that areused being weaker, it is necessary to indicate their value in lens having a length of focus of four inches would be of but one-fourth the power of the unit, one of inches focus should betermed a onethirty-sixth. From this method it has become custom-ary to speak of a number ten lens, meaning really a one-tenth focalpower; a number twenty-four is a one twenty-fourth; the larger num-ber thus indicated the weaker power. So long as the
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