The student's guide to diseases of the eye . blespherical glasses, distant vision is less improvedthan it ought to be (supposing, of course, that noother changes are present to account for the defect).No definite rule can be laid down as to the degreeof defect which should raise the suspicion of As.;indeed, in the higher degrees of even simple M. and ASTIGMATISM 299 H., acuteness of vision is often below normal(pp. 218 and 280). As. may be measured either by trial with glasses,by retinoscopy (p. 55), or by ophthalmoscopicestimation (p. 53) of the refraction of the retinalvessels in the two chi
The student's guide to diseases of the eye . blespherical glasses, distant vision is less improvedthan it ought to be (supposing, of course, that noother changes are present to account for the defect).No definite rule can be laid down as to the degreeof defect which should raise the suspicion of As.;indeed, in the higher degrees of even simple M. and ASTIGMATISM 299 H., acuteness of vision is often below normal(pp. 218 and 280). As. may be measured either by trial with glasses,by retinoscopy (p. 55), or by ophthalmoscopicestimation (p. 53) of the refraction of the retinalvessels in the two chief meridians. A comparativelyeasy qualitative test is found in the apparent shapeof the disc, which instead of being round, is more orless oval. In the erect image the long axis of theoval corresponds to the meridian of greatest refrac-tion, and is therefore as a rule nearly vertical(Pig. 110). In the inverted image (Fig. Ill) the direction ofthe oval is at right angles to the above, providedthat the object lens be nearer than its own focal.
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