Among 4000 cases of ocular headaches, 73 per cent. had astigmatism. Patients with high degrees of astigmatism are apt to go through life without complaining of eye-strain, while a larger number of persons suffer who have only the lowest degrees of astigmatism. This may be partly because low degrees of astigma tism are more common than the high degrees; and probably, also, because those with high astigmatism early learn to save their eyes, while persons who are able to see perfectly use and abuse their eyes more freely. W. F. Mitten dorf (Trans. Amer. Oph. Soc., '95).
In recent examination of personal case book it was noted that a large number of cases that had been examined a few years ago recently on re-examination showed marked changes in the astigma tism. In some cases a change from em metropia to myopic astigmatism had oc curred, others of gradually increasing myopic astigmatism, and still a third class in which there had been an in crease in the hypermetropic astigmatism. Hotz (Phila. Med. Jour., July 10, '98).
Many observers believe that astigma tism against the rule, or astigmatism with the meridian placed obliquely, causes more annoyance than astigma tism of the usual form, in which the meridian of greatest refraction is ver tical. This latter may be due to the fact that the astigmatic eye can see perfectly only the lines that run in the direction of one of its principal merid ians, and that most of the lines which we wish to distinguish are either vertical or horizontal.
The eye with oblique astigmatism sees vertical and horizontal lines rotated, and for the two eyes to work together in binocular vision this rotation of the images must be compensated by rotation of the eyeball through symmetrical ac tion of the oblique muscles. Such an action may cause symptoms of eye-strain. G. C. Savage (Oph. Record, July, '91).
In any given cases of astigmatism lines perpendicular to each other have their images rotated in opposite directions. It is, therefore, impossible by rotation of the eyeball to make all the lines of the image in one eye correspond to the lines of the image in the other. Compensatory rotation of the eye is, therefore, only necessary when the lines running in some one direction are decidedly pre dominant over lines running in other 1- directions. H. Wilson (Arch. of Oph., July, '94).
Uncorrected astigmatism has been re garded as interfering with the use of various optical instruments.
A careful consideration of the optical theory of the microscope shows that, on account of the "penetrating power" of that instrument, astigmatism interferes but little with its use. Only when the astigmatism is of high degree and a low power of microscope is employed is it necessary to consider it at all, or resort to correcting lenses. With the high powers of the microscope even the high grades of astigmatism cause no distor tion or blurring of the image. Woodruff (Jour. Amer. Med. Assoc., Nov. 24, '94).
Etiology. — Astigmatism is caused by a lack of symmetry in the curvature of the refracting surfaces of the cornea or crystalline lens, or an oblique position of such surfaces with reference to the visual line. The statement still some times made, that obliquity or distortion of the retina is capable of causing astig matism, merely betrays the ignorance, of him who makes it, of the nature of the defect. Astigmatism caused by the cornea may be partly or wholly corrected by an opposite astigmatism caused by the crystalline lens. The wide use of the keratometer (ophthalmometer) of Javal has furnished extended statistics regard ing corneal astigmatism, which, by com parison with the total astigmatism of the eye, also indicates the astigmatism due to the crystalline lens.
Examination of 500 eyes with the ophthalmometer showed that in 6 per cent. of all cases the corneal astigma tism corresponded with the total astig matism both as to amount and as to the direction of the principal meridians. In 16.6 per cent. additional the amount of corneal astigmatism exactly equals the total. and in 41.6 per cent. the difference equals 0.50 dioptre or less. Of the 77.4 per cent. in which corneal astigmatism does not correspond with the total, the -34 farmer is in excess in 62 per cent. and the latter in 15.4 per cent. In 34.6 per cent. the instrument indicates the direc tion of the total astigmatism. E. Jack son (Annals of Oph. and Otol., Oct., '94).
In 150 eyes affected with astigmatism the amount measured by the oplithal mometer was greater by 0.50 dioptre, on an average, than that found subjectively by glasses, during paralysis of the ac commodation by atropine or scopola mine. Andogsky and Dolganoff (Ann. d'Ocul., Nov., '94).