SPECIFICATION. 



SPECTACLES. 



TOO 



attended with the evolution of heat, sufficient under proper arrange- 

 ment* to ignite amadou, or German tinder. On rarefying air an oppo- 

 aite effect u observed, a* U evident from the dimness aeen on the inner 

 surface of an air-pump receiver on first beginning to exhaust the air. 

 The capacity for heat of the remaining air i increased, and there ia 

 not sufficient heat to retain the vapour in its elastic form. So when 

 compresMxl air suddenly escapes, it abaorbi heat, and moisture U 

 depoaited. Similar effect* may be observed in nature. A blast of 

 cold air descending from a lofty height, has its temperature raised by 

 the mere compression that it undergoes. 



A body in the liquid state has a higher specific heat than the same 

 substance in a solid form. The specific heat of water, for example, 

 is double that of ice. Indeed the large specific heat of water has a 

 great effect on the temperature of the globe, in moderating the rapidity 

 of the transitions which would otherwise occur. 



The determination of the specific beats of gases and vapours is 

 attended with difficulty. Regnault's results are probably very near 

 the truth. Taking the specific heat of an equal weight of water as the 

 unit of comparison, he finds that of air to be = 0-2377. The following 

 are a few of his results : 



There is a remarkable relation between the specific heat of an ele- 

 mentary body and its chemical equivalent. If the specific heat of a 

 body in the solid state be multiplied into the chemical equivalent of 

 the same body, it gives a number which coincides almost exactly with 

 the product obtained by multiplying together the specific heat and the 

 equivalent of any other elementary substance. 



SPECIFICATION. [PATEKT.J 



SPECTACLES are lenses so mounted in frames as to be conveniently 

 held before the eyes to assist defective vision. The eyes of a person 

 whose sight is much tried often receive injury from delay in the use of 

 spectacles ; while the sight of many persons is prematurely worn out 

 by the use of glasses of too high a power. Whether the glasses used 

 be concave or convex, the lowest power that is available should be 

 used. The use of a single reading-glass instead of spectacles is in- 

 jurious, since, by occasioning one eye to be more used than the other, 

 the power and focal length of the two are rendered unequal. The 

 unsteadiness of the glass is also a disadvantage. The defects of the 

 single hand-glass are not removed by increasing its size so much that 

 both eyes may see through it, because in that case the axis of each 

 pencil of rays will be distorted by passing through the lens at points 

 beside its centre. Hand-spectacles, which are made to fold up into 

 nearly as small a space as a single glass, are better than reading- 

 glasses ; but, although steadied in some degree by reeling upon the 

 nose, they are not equal to spectacles well fitted to and supported by 

 the head. 



Varieties in the conformation of the eyes, and in the manner and 

 degree in which they are affected by use, render it impossible to lay 

 down any rule for the focal length of convex glasses for persons of a 

 given age ; yet a general idea of the necessary power may be obtained 

 from the following table, extracted from Dr. Kitchener's ' Economy of 

 the Eyes:' 



Inches of focus. 



10 

 

 8 

 7 

 6 



In some cases it is advisable to use different spectacles for night and 

 day, to suit different degrees of light; and it is, generally speaking, 

 well to increase the power of the glasses as the sigh weaker 



from age. Even in this, however, caution must be exercised, lest, by 

 over-stimulating the eye, its powers be too rapidly exhausted. It 

 should always be borne in mind, both by young and old persons, and in 

 changing spectacles M wi-11 as in first taking to them, that the legiti- 



mate object of convex spectacles is to restore the natural power (or 

 rather the natural fociu) of the eye, and not to enable the wearer to 

 see objects larger or more distinctly than with the eye in a 

 and healthy condition. Short-sightedness being still leas ilq 

 upon age, cannot be met by any rule even so well as the opposite 

 .1. ( 



In choosing spectacles, the frame should fit comfortably to the head, 

 and be of such a form as to bring the centre of each lens exactly oppo- 

 site to the centre of the eye. Should this not be the cose, the bridge 

 or wire that unites the two halves of the frame may be stretched or 

 bent to suit the wearer. 



With respect to the glass used in spectacles, it may be taken as a rule 

 that, except in cases where it is necessary to protect the eyes from an 

 injurious glare of light, the most colourless material is to be preferred. 

 The accurate figure of the lenses cannot be too strongly iusi.-- 

 and may be tested by holding the spectacles near to a printed book, 

 and gradually moving them nearer to the eye ; by doing which, if the 

 glasses be not well-shaped, the letters will appear distorted. Veins or 

 blebs in the glass are injurious, and may be detected by holding t In- 

 glass between the eye and the flame of a candle, and moving it back- 

 wards ami forwards, until it appears full of light; wln-u uvery such 

 defect will be distinctly seen. Scratches are not quite so injurious, 

 inasmuch as they do not produce distortion; yet they shouM be 

 avoided. Their diminished liability to injury by scratching forii 

 principal recommendation of spectacle lenses of rock crystal ; but some 

 opticians consider their use injurious, owing to their tendency to 

 irregular refraction of the ravs of light. 



Several deviations from the ordinary mode of constructing specta- 

 cles may be alluded to. The pcritcopic spectacles of Dr. Wollaston 

 were contrived in order to allow considerable latitude of motion to the 

 eyes without fatigue, by conforming the shape of the glasses to th.it 

 of the eyes. This is effected by the use of lenses either of a meniscus 

 or concavo-convex form : the concave side being in both coses turned 

 towards the eye. fig. I represents in section, the form of the lens 



Fig. l. 



Fig. 2. 



used in convex or magnifying spectacles of the periscopic construction ; 

 and /'<;/. 2 shows that of a concave lens suitable for short-sighted 

 persons. In the former case the curve of least radius is that of the 

 anterior, and in the- latter, that of the posterior surface of the lens. 

 Divided spectacles, each glass consisting of two half-lenses, are some- 

 times used ; the upper half of each glass being occupied by a concave 

 lens, or one of very slight convexity, for seeing distant objects, while 

 the lower half has a strong magnifier, for examining things near the 

 eye. Such spectacles have an awkward appearance, on account of the 

 joint along the middle, and require some practice to avoid inconve- 

 nience to the wearer ; but they have been used with advantage by 

 artists and others requiring to look alternately at near and distant 

 objects. Other plans have been tried for obtaining a similar advantage ; 

 such as having a second pair of glasses hinged to the frann 

 capable of either turning up out of the way, or being placed inim. - 

 diately before the ordinary lenses, to modify their power; or having 

 two distinct pairs of spectacles, capable of being used either so|>arately 

 or together. Spectacles with glazed wings, or frames partly filled with 

 crape, ore sometimes used by travellers ; the glasses, which may bo 

 plain, unless otherwise required, being of such a form as to shield the 

 eyes from dust. 



In the Great Exhibition of 1851 the British exhibitors of spectacles 

 were contented with improvements in the method of mounting, but 

 left out of sight the main object, the improvement of the glasses. 

 Thus there were fine and delicate frames, some weighing only 11 grains, 

 and the whole weight, including glasses, not amounting to more than 

 two pennyweights. The steel frames of these spectacles resembled 

 hair' lines, and were imperceptible at a short distance. Thri. 

 also ventilating eye-shades, designed to allow a current of air to 

 circulate freely between the shade* and the wearer : there were also 

 spectacles for sketching, mounted without a rim to prevent obstruction 

 of vision. These, with every variety of gold, silver, and steel-mounted 

 spectacles, were to be found in abundance ; but it was left to the 

 French exhibitors to present glasses to suit almost every pi-mliarity of 

 vi.-ion. M. Henri, uniting the skill of the optician with the knov 

 of the physiologist, exhibited spectacles of an improved kind, the glass 

 of which was so pure as to remove one source of uneasiness and 

 fatigue incident upon the use of spectacles ; while the foci were so 

 arranged as to rectify certain defects and obliquity of vision. Some of 

 these spectacles were furnished with a moveable diaphragm, to bo 

 shifted right or left at will. These were designed for the cure uf 

 squinting, either converging or diverging. M. Pouillot exhibited 



