18 THE MICROSCOPE. 



being increased, or rather strengthened, by the judicious 

 exercise, and at will, of a power termed accommodation. 

 The near-point of useful sight is fixed, for the normal 

 eye, at about ten inches from the object. It is for this 

 reason that English opticians have taken an arbitrary 

 measurement of ten inches for the length of the body 

 of the microscope. The most distant point of distinct 

 vision is placed where the image of the object falls 

 exactly on the most sensitive spot of the retina, 

 termed the far-point of vision. When the eye is 

 accommodated for viewing a near'object, the curvature 

 of the lens is slightly changed, and its front surface 

 approaches somewhat closer to the cornea. The range 

 of the field of vision is computed to be about 160 

 degrees on the horizontal plane, and 120 degrees in the 

 vertical. The eye is perfectly adjusted for parallel 

 rays of light, but when it has to do with divergent 

 rays it is frequently found unequal to the task of 

 uniting them. The great mobility of the eye-ball, 

 however, almost wholly compensates for this slight 

 defect; and, practically, all rays are parallel which 

 proceed from distant objects, that is from objects at 

 twenty feet or upwards. 



Good visual accommodation depends upon three 

 causes : 1st, changes in the indices of refraction of 

 the media (cornea, lens, &c.) ; 2nd, displacement of the 

 surface of projection (the retina, analogous to the arti- 

 ficial production of accommodation by the adjustment 

 of the camera obscura) ; and 3rd, alteration in the 

 forms of the refracting surfaces. 



Physicists assure us that the organ of vision, hereto 

 fore regarded as the most wonderful instance of creative 

 wisdom, is not perfectly achromatic ; that, in fact, 

 it possesses no proper provision for the correction of 

 its own chromatic and spherical aberrations, nor 

 for the correction of the chromatic aberration arising 

 from defects as an optical instrument, nor that arising 

 from the compound nature of light, the rays of which, 

 it is known, are refracted in different degrees and inten- 

 sities a defect slightly exaggerated by defective cen- 

 tring of the refractive surfaces of the internal eye. The 



