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XLVI. On Shadow Optometers. By John C. Douglas, East- 

 India Government Telegraph Dapartment*. 



rTHHE well-known experiment of Father Sheiner, in which a 

 JL small object is regarded through two small orifices, is the 

 principle upon which almost all optometers are constructed. 

 The distance obtained is generally considered the mean distance of 

 distinct vision. Repeated experiments do not give similar results 

 with the same person ; and the use of the principle is stated to 

 be only possibly of value when great practice has given the fa- 

 culty of altering accommodation f. The variation between the 

 results of several determinations is not such as to destroy the 

 value of these results ; a variation might be expected. The ob- 

 jection most fatal to its employment appears to me to be the 

 indefinite nature of these results. In using Sheiner's method to 

 examine the nature and position of images on the retina, and to 

 find the conjugate foci under different conditions of accommoda- 

 tion and refraction, the interposition of the diaphragm is a con- 

 dition which removes the circumstances of the case from the 

 conditions of natural vision. I venture to propose the following 

 as far preferable to Sheiner's experiment for optometric purposes. 

 If a small object be moved to and fro between the eye and 

 two or more luminous points not having conjugate foci on the 

 retina, and subtending a small angle with each other at the eye, 

 as many objects or rather shadows will be seen as luminous 

 points employed, except at a particular distance, at which the 

 object appears single : pin-holes in a card held before a lamp or 

 distant street-lamps may be used to furnish the luminous points, 

 and a common pin or pen-nib may be employed as the object. 

 The cause of this multiplication of the object is evident; the 

 objects seen are shadows of the object employed, one shadow 

 being given by each light. As there is only one object, if this 

 were placed in a focus of the eye having its conjugate on the 

 retina, it would appear single and distinct. Of the infinite 

 number of parallel planes of the pencils between the eye and the 

 sources of light only one is at a focus having its conjugate on 

 the retina in any given state of accommodation. If the object be 

 in this plane, as the section of the pencils through this plane is 

 accurately reproduced on the retina, the images of the several 

 shadows will coincide and one only be seen ; but if the object be 

 placed on any other plane the several shadows will no longer co- 

 incide on the retina, and a separate shadow due to each light will 

 be seen. A method is thus furnished for finding for what dis- 

 tance the eye is accommodated without the disturbance due to the 

 interposition of a diaphragm between the object and the eye. 

 * Communicated by the Author. f Donders. 



