COMMON THINGS SPECTACLES. 



Whatever be the defects of sight which they may he used to 

 remove, it is evident that the lenses ought to be so mounted that 

 their axes shall be parallel, and that their centres shall coincide 

 with the centres of the pupils when the optical axes are directed 

 perpendicular to the general plane of the face, that is to say, when 

 the eyes look straight forward. 



These conditions, though important, are rarely attended to in. 

 the choice of spectacles. If spectacles be mounted in extremely 

 light and flexible frames, the lenses almost invariably lose their 

 parallelism, and their axes not only cease to be parallel, but are 

 frequently in different planes. Spectacles ought therefore to be 

 constructed with mounting sufficiently strong to prevent this 

 derangement of the axes of the lenses, and in their original 

 construction care should be taken that the axes of the lenses be 

 truly parallel. 



In the adaptation of spectacles it is necessary that the distance 

 between the centres of the lenses should be precisely equal to the 

 distance between the centres of the pupils. The clearest vision 

 being obtained by looking through the centres of the lenses, the 

 eyes have a constant tendency to look in that direction. Now if 

 the distance between the centres of the lenses be greater than the 

 distance between the centres of the pupils, the eyes having a ten- 

 dency to look through the centres of the lenses, their axes will 

 cease to be parallel, and will diverge as in the case of an outsquint. 

 On the other hand, if the distance between the centres of the 

 lenses be less than the distance between the centres of the 

 pupils, there will, for a like reason, be a tendency to produce 

 an insquint. 



I have myself known persons of defective sight, who had never 

 been able to suit themselves with spectacles, and concluded that 

 they had some defect which spectacles could not remedy. Upon 

 observing the form of their heads, I found, in each case, that the 

 eyes were more distant asunder than eyes generally are, while 

 the spectacles they used, being those made with the lenses at the 

 usual distance, were never, and never could be, so placed as to be 

 concentrical with the eyes, and hence arose the discomfort attend- 

 ing their use. In all such cases I removed the inconvenience by 

 measuring the distance between the centres of the eyes, and caus- 

 ing proper glasses to be mounted in frames, so that the distance 

 between their centres should correspond with the distance between 

 the centres of the eyes. 



I would therefore advise every one who uses spectacles to cause 

 the distance between the centres of their eyes to be exactly mea- 

 sured, and to select for their spectacles mountings corresponding 

 with this distance. 

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