The human eye also belongs to a relatively small group that is equipped to identify different colours. Most insects, fish, birds and many animals can only distinguish different shades and textures. The basis of colour sense is the mixture of three transparent colours — red, green and blue. When they are combined in the correct intensities they make up white. People with a well-developed colour sense can recognize many hues of the same colour, perhaps even as many as a hundred. But colour sense is highly variable, and it is estimated that one male in eight is `colour-blind‘; that is to say, Read the rest of this entry »
These then are the common errors of vision which may cause eye-strain. They are short- and long-sightedness, near-vision deficiency and astigmatism. Most commonly astigmatism is present with one of the other three conditions. They may all be measured by both objective and subjective methods. Objective measurement means the use of instruments, while subjective measurement requires you to state an opinion. In ordinary practice the practitioner will use both methods, and thereby establish a pattern which enables him to prescribe the right optical correction. If you repeatedly give a wrong answer to a subjective inquiry the experienced occulist will proceed to further objective (optometrical) tests. Read the rest of this entry »
The symptoms of eye-strain are diverse and confusing. Inability to use the eyes for a specific task leads to frustration in most intelligent people, regardless of whether such inability is temporary or permanent. There are states where the individual finds that, by squeezing the eyes, or by holding objects at abnormal distances, better vision is possible, but this only leads to fatigue and congestion of the delicate musculature. Read the rest of this entry »
As regards lens-implant surgery the benefits are very great. To be able to have normal sight again without the need for thick spectacles or the bother of wearing contact lenses is surely the preferred treatment. Where contact lenses are used subsequent to a cataract operation, they differ from ordinary contact lenses in certain ways. The condition of an eye that has had its crystalline lens removed is known as aphakia, and aphakia almost invariably demands a contact lens that is much thicker in its centre than the lens worn by the ordinary short-sighted person. Read the rest of this entry »
The results of these preventive measures were disappointing. Some observers reported a slight decrease in the percentage of myopia in schools in which the prescribed reforms had been made, but on the whole the injurious effects of the educational process were not eliminated to any extent.
Further study of the subject has only added to its difficulty, while at the same time it has tended to relieve the schools of much of the responsibility ‘formerly attributed to them for the production of myopia. As the American Encyclopedia of Ophthalmology points out, “the theory that myopia is due to close work aggravated by town life and badly lighted rooms is gradually giving ground before statistics.” Read the rest of this entry »
No phase of ophthalmology, not even the problem of ac- commodation, has been the subject of so much investigation and discussion as the cause and prevention of myopia. Since hypermetropia was supposed to be due to a congenital deformation of the eyeball, and until fairly recently astigmatism was also supposed to be congenital in most cases, these conditions were not thought to call for any explanation or to admit of any prevention; but myopia appeared to be acquired. It therefore presented a problem of immense practical importance to which many eminent men devoted years of labor. Read the rest of this entry »
It is also obvious that the method must have prevented other errors of refraction, a problem which previously had not even been seriously considered, because hypermetropia is supposed to be congenital and until not long ago astigmatism was also supposed to be congenital in the great majority of cases. Anyone who knows how to use a retinoscope, however, can demonstrate in a few minutes that both of these conditions are acquired; for no matter how astigmatic or hypermetropic an eye may be, its vision always becomes normal when it looks at a blank surface without trying to see. Read the rest of this entry »
To repeat a very important principle: you cannot see anything with perfect sight unless you have seen it before. When the eye looks at an unfamiliar object it always strains more or less to see that object, and an error of refraction is always produced. When children look at unfamiliar writing or figures on the blackboard, distant maps, diagrams, or pictures, the retinoscope always shows that they are myopic, though their vision may be absolutely normal under other circumstances. The same thing happens when adults look at unfamiliar distant objects. When the eye regards a familiar object, however, the effect is quite different. Not only can it be regarded without strain, but the strain of looking at unfamiliar objects later is lessened. Read the rest of this entry »
A shortsighted young woman, to take the opposite of this case, had a passion for mathematics and anatomy and excelled in those subjects. She learned to use the ophthalmoscope as easily as the farsighted girl had learned Latin. Almost immediately she saw the optic nerve and noted that the center was whiter than the periphery. She saw the light-colored lines, the arteries; and the darker ones, the veins; and she saw the light streaks on the blood vessels. Some specialists never become able to do this, and no one could do it without normal vision. Her vision, therefore, must have been temporarily normal when she did it. Her vision for figures, although not normal, was better than for letters. Read the rest of this entry »
