COMMON THINGS SPECTACLES. 



vidual have exactly the same refracting power. That this is the 

 case is evident, from the fact that the lenses provided in the same 

 spectacles have invariably the same focal length. 



Now although it is generally true that the two eyes in the 

 same individual have the same refractive power, it is not invari- 

 ably so ; and if it be not, it is evident that lenses of equal focal 

 length cannot be at once adapted to both eyes. 



"When the difference of the refractive power of the two eyes i 

 not great (which is generally the case when a difference exists 

 at all), this inequality is not perceived. By an instinctive act of 

 the mind, of which we are unconscious, the perception obtained by 

 the more perfect of the two eyes in case of inequality is that to 

 which our attention is directed, the impression on the more defec- 

 tive eye not being perceived. 



It might be expected, however, that the inequality woulc. 

 become apparent, by looking alternately at the same object with 

 each of the eyes, closing the other ; but it is so difficult to compare 

 the powers of vision of the two eyes when they are not very 

 unequal, by objects contemplated at different times, even though 

 they should be exhibited in immediate succession, that this 

 method fails. 



18. My attention having been directed to this question, I 

 contrived an apparatus, which may not inaptly be called an 

 Ophthalmometcr, by which the least difference in the powers of 

 vision of the two eyes may be rendered immediately apparent. 



The principle I adopted for this purpose, resembles that which 

 has been otherwise applied with success in photometers. I have so 

 arranged the apparatus, that two similar objects similarly illumi- 

 nated shall be at the same time visible in immediate juxtaposition, 

 the one by the right eye being invisible to the left eye, and the 

 other by the left eye being invisible to the right eye. 



This apparatus consists of a small box, A B c D, fig. 5, about 

 five inches in width, A D, ten inches in length, A B, and six inches 

 in height. "Within this there slides another box, A' B' c' D', made 

 nearly to fit it, but to move freely within it, the interior of this 

 box being blackened, or lined with black velvet. In the end, B' c', 

 is a rectangular aperture, the length of which M N is about an 

 inch, and the height about half an inch ; the length, however, 

 being capable of being augmented and diminished by slides. 

 Opposite to the end of the box B c is a white screen, on which is 

 traced a horizontal line parallel and opposite to the opening M N, 

 and marked with a divided scale, the of which is opposite to the 

 centre of the aperture M x, and the divisions upon which are 

 numbered in each direction from by 1, 2, 3, 4, 5, 6 



Let us suppose the eyes now applied at B. and L. Let the 

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