464 REPORT— 1880. 



(1) The patient's sensations may be entirely disregarded, or only used as con- 

 firmatory. 



(2) The image used is necessarily at the retina, and not before or behind it. 



(3) The accommodation, or any defects in the refraction of the observer's eye, 

 does not enter into the result, as the only function of this eye is to observe the 

 formation of the image on the retina. 



In the first plan a lamp I is provided with a condensing lens c, and a series 

 of radiating wires iv (supposed to be seen edgeways in the figure), thus giving a 

 bright field with black lines on it. 



The whole slides on a graduated bar, C, at the other end of which is a convex 

 lens,?/ [4 and 10 dioptrics are the most convenient powers, i.e. 10 and 4 inch focus]. 

 Close to the lens, and at an angle of 4;5° to its axis, is a plane mirror (»»), wliich 

 reflects the rays at riijht angles to their former path. The instrument is to be 

 held so that this pencil of rays enters the observed eye, and when the wire screen 

 is at the proper distance, an image of it is formed on the retina. The mirror has 

 the centre left unsilvered, as in an ordinary ophthalmoscope, and has a disc of 

 correcting lenses behind it, to render the retina, and the image on it, visible by the 

 direct method. The observed eye should have its accommodation relaxed by 

 atropine. 



The bar is so divided that when an image of the whole or part of the screen 

 is sharp on the retina, the graduation expresses the refractive error of the corre- 

 sponding meridian. Hence, if the image of the whole screen is seen to be equally 

 sharp, the eye is known to be not astigmatic, and the graduation gives the number 

 of dioptrics by which it is myopic or hypermetropic. If the lines be not aU 

 equally sharp, then the most distant point at which a distinct image of any 

 of the wires is formed on the retina gi\"es the refractive error of the meridian 

 of minimum refraction expressed in dioptrics, and the point at which the line 

 at right angles to this is best defined, gives that of the meridian of maximum 

 refraction. The least of these gives the spherical element of the correcting lens 

 required for distant objects, and the difference between the two gives that of the 

 cylindiical part. The meridian of maximum refraction is that in which the line is 

 visible when the wires are at the greatest distance. 



In the second plan the lamp, /, condensing lens, c, and wire screen, w, are 

 similar, and only differ in size, the front lens, y, and mirror, in, are also similar, 

 but the lamp and wires are permanently fixed by a tube, so that the wires are 

 accurately in the principal focus of the front lens, y. By this means the rays 

 from the wires (or rather from the interval between them), after refraction through 

 the lens and reflection by the mirror, are parallel. If received by an eye which is 

 emmetropic, and witli its accommodation relaxed, an image of the wires is formed 

 on the retina. The light radiating from this image passes out through the optical 

 system of the eye ; is rendered parallel and able to form an accurate image on the 

 retina of an emmetropic eye observing through the hole in the mirror. 



If the observed eye be not emmetropic, it is only necessary to introduce lenses 

 of different powers close in front of it, so as to correct the rays both entering and 

 leaving the eye. If the refraction be the same in all meridians, the image of all 

 the wires is sharp with the same lens, and this lens is the one required to correct 

 the ametropia. If any astigmatism exists, different lenses are required for render- 

 ing the images of the diflferent wires sharp. 



The strongest and weakest of these are the measures of the errors of refraction 

 of the two principal meridians, and the diff'erence of their numbers of dioptrics 

 gives the cylindrical element of the correcting glass required. 



In this form of apparatus a disc of correcting lenses behind the mirror is not 

 required, as the single correcting lens near the observed eye corrects the rays both, 

 entering and leaving the ej'e. 



For rapidly finding the proper lens a disc of lenses is used, each a centimetre 

 in diameter, and with intervals of one dioptric ; a smaller disc is attached contain- 

 ing the quarter dioptrics, so that by their combination intervals of one quarter of a 

 dioptric can be read — a degree of accuracy greater than the estimation is generally 

 susceptible of. 



