212 NEGRETTI AND ZAMBRA, HOLBORN VIADUCT, E.G., 



OPTICAL INSTEUMENTS. 



THE science of .Optics, which consists in the examination of the phenomena of light 

 and vision, is one of the most important and most useful branches of physical 

 science. By the aid of its appliances we are permitted to obtain a glimpse' of the 

 immensity of the universe, and are enabled to reveal wonders of creation, of which 

 but for this power granted to us we should be in perfect ignorance. By means of 

 the Telescope we are made acquainted with the existence of spheres and worlds 

 floating in boundless space, illustrating in the most sublime manner the perfect 

 harmony that exists in the motions of the heavenly bodies. 



The Microscope affords an insight into the minute structure of animal and 

 vegetable life, and discloses to the wondering spectator forms of life, the variety 

 and beauty of which display in the most convincing manner the infinite power of 

 the Great Creator. 



But of all the gifts which science has so freely lavished on humanity, and all 

 tending to its benefit and improvement, perhaps there is none that can rank higher 

 than the means afforded of assisting the natural vision, and of enabling us to correct 

 in a most simple and perfect manner the irregularities of sight, which are conse- 

 quent on alterations silently going on in the structure of that wonderful and deli- 

 cately constructed organ, the Eye. The sight has in all time been justly accounted 

 the greatest of blessings, and it deserves our strictest attention in order that the 

 advantages of it may not be lost to us at an earlier period than is absolutely 

 necessary from physical decay. 



Those beginning to require the aid of Spectacles are obliged, before distinct 

 vision can be obtained, to hold the candle or to have the source of light 

 between the eye and the book they read, in order to force the pupils of their eyes 

 into a proper state of contraction, that they may see distinctly the characters 

 before them. Now this is a state of things that should never occur, for if indulged 

 in, and the eye be tampered with, it will eventually lead to great impairment of 

 vision. The power of adjustment in the eyes varies exceedingly in different 

 individuals and also at different periods in the life of each person ; being strongest 

 in youth, and gradually diminishing with advancing years. 



From this circumstance it is easy to see the reason of the fatigue caused by 

 the strain on the ciliary process of the eye in bringing it to a proper adjustment 

 for objects at different distances, and an individual who has habitually to make an 

 effort to adjust his eye to these variations of circumstances, should lose no time in 

 applying to the Optician to obtain assistance from the use of glasses. 



From what we have said above, let it not be supposed that the indiscriminate 

 use of Spectacles is recommended ; very far from it. We must, before resorting 

 to Spectacles, ascertain the nature of the defect in the visual organs, and then 

 have the amount, and only the exact amount of correction applied ; just in the 

 same manner as with a telescope, we are obliged to draw out the eye-tube until 

 a perfect image appears in the field of view, nothing more or less will suffice to 

 this end. 



