14 THE AMERICAN MONTHLY [Jan. 



aperture than lie could with the achromatic systems of 

 small aperture of that day. And since that time there 

 has been an ever-increasing desire to enlarge the aper- 

 ture of objectives. It was soon found out, however, 

 that with every degree of enlarged aperture attempted, 

 the delicacy of the aberrations to be corrected were also 

 enormously increased. So that the computation of a 

 modern objective is truly a gigantic undertaking. 



Some years later a fluid was suggested to be used be- 

 tween the anterior lens and the cover glass, to prevent 

 the gr 3at loss of liglit due to refraction. And I believe 

 Mr. Toll.es constructed lenses with which he used balsam 

 as the immersing medium. Before this, however, Powell 

 and Lealand had constructed very fine water immersion 

 lenses. Since Dr. Abbe's adoption of cedar oil this has 

 generally come into use, and other immersion fluids, 

 some of which possibly have sj)ecial uses, have been grad- 

 ually discontinued. Cedar oil was adopted by Dr. Abbe 

 only after exhaustive experiment, and it stands without 

 a rival in dispersive suitability. 



For the purpose of vision an image must be formed on 

 the retina of the eye, and the purpose of the compound 

 microscope is to form an enlarged image of minute 

 structures that are indistinguishable by the unaided eye. 

 Images are formed by rays, and every single ray has 

 the power of forming an image. If such were not the 

 case the lens would have no image-forming power itself. 



To show this make the pinhole experiment. Light a 

 candle in a dark room. Arrange a sheet of cardboard 

 two feet distant, and between the two put a blackened 

 card punctured with a pinhole. We see then an inverted 

 image of the flame on the white card. If we make the 

 hole in the card larger, we get a brighter but no longer 

 a sharp image, because now it is really a number of 

 images, formed by rays that cross at different points of 



