VITAL STATISTICS. 



1469 



was presbyopia for horizontal lines and 

 myopic for vertical. This lie remedied by 

 wearing spectacles the glasses of which were 

 cvlindric bi-convexes, with rectilinear, hori- 

 zontal and similar axes. These glasses obvi- 

 ated the presbyopia relative to the horizontal 

 lines, and they were combined with sphero- 

 biconcave lenses to get rid of the myopia for 

 vertical lines. Each of the glasses was made 

 moveable for facility of cleaning. 



The following means are recommended to 

 ascertain if an eye has the defect now de- 

 scribed. The person should attentively con- 

 template for some time and with attention a 

 cross, three or four lines in size, made of fine 

 wire and fixed in a frame. If affected, he 

 will see the horizontal lines differ in thickness 

 and blackness of tint from the vertical. 



To determine the focal length which the 

 lenses should have, a person whose sight is 

 presbyopia in one direction should take bi- 

 sphero-convex lenses which enable him to see 

 distinctly at the ordinary distance the lines 

 which otherwise appear indistinct: he can 

 deduce the focal distance of the cylindrico- 

 convex glasses. A person myopic in one 

 direction should do the same with regard to 

 bi-sphero-concave lenses. The convex glasses 

 should be chosen of one or two numbers 

 stronger. 



BIBLIOGRAPHY. Baptista Porta, Magire Natu- 

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 pendente, De Yisione. Venetiis, 1600. Aqiti/oi/itts, 

 Opticorum, libri iv., Antverpise, 1013. St-hi'titcr, 

 Oculus, /Eniponti, 1619. De la Hire, Me'moires de 

 Mathe'matiqiie, &c. Paris, 1694. Gregorian, Optica 

 Promota, seu Radiorum Hysteria euucleata. Ma- 

 seres, Script. Optici. Barrow, J., Lectiones XVIII. 

 in quibus Opticorum Phenomenon Eationes ex- 

 ])ouiintiir. Maseres, Script. Optici. Berkeley, A New 

 Theory of Vision, Dublin, 1709. Mariotte, CEuvres, 

 Leide, 1717. Pemberton, De Facilitate Oculi. Haller, 

 Disp. Anat. vol. vii. 1719. Jitrin, An Essay on 

 Distinct and Indistinct Vision, Smith's Complrat 

 System of Opticks, vol. ii. 1738. PorterfiM, W M.D., 

 A Treatise on the Eye, 8vo., Edin., 1759. Fontumt, 

 Dei Moti deH'Iride, Lucca, 1765. Priestley, J., The 

 History and Present State of Discoveries relating to 

 Vision, Light, and Colours, quarto, London, 1772. 

 Harris, A Treatise of Optics. London, 1775. Oll>ers, 

 De Internis Oculi Hutationibus, Gott. 1780. Blu- 

 menbach, De Oculis Leucrethiopum et Iridis Motu, 

 1786. Mashelyne, Phil. Trans. 179. Hossaca, Phil. 

 Trans. 1794. Ramscitn, Phil. Trans. 1795. Du 

 Tour, Mem. de Mathe'matique et de Physique, 

 t. iii. and iv. Dalton J., Extraordinary Facts re- 

 lating to the Vision of Colours. Memoirs of the 

 Literary and Philosophical Society of Manchester, 

 vol. v. pt. i. 1798. C/ie?ievi.r, Phil. Trans. 1*03. 

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 1807, and in Philosophical Transactions. Gerson, 

 De Forma Cornea;, Gott. 1810. Horn, On the Seat 

 of Vision, 1813. Wells, Essay on Single Vision 

 with Two Eyes, London, 1818. Chossat, Annales de 

 Chimie, Paris, 1819. Gillies, J., Account of a Pecu- 

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gazine, and in Edinb. Philos. Journal. Bert holt I, 

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 1830. Tourtual, Die Chromasie des Auges, Meek. 

 Arch. 1830. Rosas, Handbuch der Theoretischen 

 und Practischen Augenheilkunde, Wien, 1830. 

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 diversam Distantiam Objecti, Traj. ad Rh. 1832. 

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 Trans. Royal Society of Edinburgh, vol. xiii. 1836. 

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1839. Herschel, Sir J. fT., article " Light," Cyclo- 

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1840. Mackenzie, W., M.D., The Physiology of 

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(The subject of Vision is more or less treated of in 

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 and are to be found in Philosophical Transactions 

 and Journals, in the Annales d'Oculistique, &c.) 



(W. White Cooper.) 



VITAL STATISTICS. The duration of 

 human life, with a consideration of the princi- 

 pal causes by which it may be lengthened or 

 curtailed, is a subject which evidently belongs 

 both to the domain of physiology and to that 

 of statistics. It belongs to physiology, in- 

 asmuch as the duration of human life is the 

 final effect of the operation of natural causes 

 brought to bear on the healthy human frame ; 

 and it belongs to statistics, whether we use 

 that term in the less exact sense of a branch 

 of human knowledge largely indebted to the 

 use of numbers, or in the more accurate sense 

 of a department of science, having an im- 

 portant bearing on the interests of the public.* 



In this place it is proposed to take only 

 a limited view of the subject of Vital Sta- 

 tistics, and to examine the scientific methods 

 which have been suggested and employed for 

 determining the true duration of human life in 

 communities and classes of men j in other 



* For some remarks on the true meaning of the 

 term "Statistics," see STATISTICS, MEDICAL, note, 

 p. 803. 



