Scientific Lectures. 37 



the lens than the red. And between these 'images will be formed 

 others bj the intermediate rays. The total number of images thus 

 formed is unlimited, and superposed upon each other their effect is 

 to produce confusion. Color will be visible only at the margin of the 

 blended images. Chromatic aberration is the second great difficulty 

 to be overcome in the construction of the microscope. It happens, 

 fortunately, that the separation of the colors (technically called 

 dispersion) by the difiereut media, is not proportioned to the refracting 

 power. As the dispersions of convex and concave lenses are in 

 opposite directions, a combination of two such lenses may be obtained, 

 in which dispersion shall be neutralized, while there still remains a 

 balance of refracting power outstanding. Thus a concave lens of 

 flint glass will correct the dispersion of a more powerfully refi'acting 

 convex lens of crown glass, and the image formed by the combination 

 will be colorless. It is then a mathematical question to find the 

 curvatures proper to be given to such lenses, so as to correct spherical 

 and chromatic aberration at the same time. 



A third great difficulty to be contended with in the construction 

 of this instrument, results from the fact that the image of a plane 

 object formed by any single lens, is not plane but curved. A flat 

 object observed through an eye-glass, seems to round up toward the 

 eye, in the center. A positive image of such an object, formed by a 

 convex lens, is hollowed out or concave, toward the lens. Moreover, 

 though the spherical aberration may be corrected for the axis of the 

 lens, it will not be so for the oblique rays. Distortion of the form 

 of tli^ object observed and confusion in the marginal portions of the 

 image will therefore still remain, even after the corrections above 

 described have been made. To conquer this last difficulty, so as to 

 produce a perfectly undistorted image and an entirely flat and clear 

 field, has exacted the largest amount of study and skill which the 

 problem has called into exercise. The complete solution is only 

 obtained by combining the eflects of all the parts of the compound 

 microscope, of which we shall presently speak. One word is first 

 necessary in regard to the phenomena of vision. Tlie eye is an opti- 

 cal instrument, in which an image is regularly formed as in the 

 camera obscura. The sensitive surface on which this image falls is 

 called the retina. The eye can accommodate itself to the distance 

 of the object, so as within limits largel}^ variable to maintain the 

 image on the retina distinct. The image on the retina must have a 

 certain size, very small no doubt, but ascertainable, in order that we 



