A treatise on the diseases of the eye.

  • Wells, J. Soelberg (John Soelberg), -1879.
Date:
1873
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on the diseases of the eye. Source: Wellcome Collection.

Provider: This material has been provided by the Gerstein Science Information Centre at the University of Toronto, through the Medical Heritage Library. The original may be consulted at the Gerstein Science Information Centre, University of Toronto.

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    —T 1 ? Doubtful if tension be less than natural. —T 1. First degree of reduced tension. Slight but positive reduction of tension. « __T 2 ) Successive degrees of reduced tension, short of such « —T 3 J considerable softness of the eye as allows the finger to sink in the coats. It is less easy to define these by words. In common practice, some of these may be regarded as refine- ments ; but in accurate note-taking, where the nature and course of various diseases of the globe are under investigation, I have found them highly serviceable, and they have as much precision as perhaps is attainable or desirable. It is also to be borne in mind that the normal tension has a certain range 'or variety in persons of different age, build, or tem- perament ; and according to varying temporary states of system as regards emptiness and repletion. Experience will make every one aware of these varieties, which do not encroach on the above abnormal grades of tension. Medical men may understand how important is this matter of the degree of tension, by considering how priceless would be the power of accurately estimating it by the touch in the case of various head affections. For the examination of the acuteness of vision various test-types are used, more especially those of Jaeger and Snellen. The former do not, however, afford a perfect clue to the acuteness of vision, for a person may be able to read No. 1 of Jaeger with facility and yet not enjoy abnormal acuteness of sight. Snellen has, however, devised a* set of test-types which fulfil this desideratum. The letters are Square, and their size increases at a definite ratio, so that each number is seen at an angle of five minutes. Thus, No. 1 is seen by a normal eye up to a distance of one foot, at an angle of five minutes, No. 2 up to two feet, and so on. These numbers cannot, as a rule, be seen distinctly beyond these distances.1 Now, if the eye is suffering from any diminution of acuteness of vision, it will require to see the letters under a larger angle than that of five minutes, in order to gain larger retinal images. No. 1 cannot be read at a distance of one foot, but only, perhaps, No. 4 or 5. We may easily calculate the degree of the acuteness of vision thus :— The utmost distance at which the types are recognized (d)divided by the distance at which they appear at an angle of five minutes (D), gives the formula for the acuteness of vision (V): Y= -jj. ' At Professor Longmore's suggestion, Dr. Snellen has given in his second edi- tion of the test-types some tables containing a series of figures and single numbers, for the examination of such recruits for the British Army as are unable For further information as to the examination of the sight of recruits, I ,™st reler the reader to Professor Longmore's excellent » Ophthalmic Mam al hich 1 would also recommend to the special notice of the surgeons of the JV ilitia an Vol- teer Corps These test-types may be obtained at Messrs. Williams and I un Henrietta-street, Co vent Garden. «*-,;«««• fr>r nor* in [A selection from the test-types of both Jaeger and Snellen, sufficient f ordinary practice, will be found at the end ot this volume.]
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