198 LECTURES TO SCIENCE TEACHERS. 



combinations chiefly due to the lenses placed at the "bottom 

 of the object-glass, and the spherical and chromatic aberrations 

 are corrected as far as possible by the lenses placed above 

 them, but as I do not suppose the ladies and gentlemen here 

 present are at all likely to make microscopes for themselves, 

 I will not occupy time by going into all the details of con- 

 struction of object-glasses. It is a most difficult subject, both 

 in theory and practice, and the more one knows of the diffi- 

 culties to be contended with in making lenses of high power 

 the more one feels astonished at the excellence of some that 

 are now made. Some of the high-power object-glasses, with 

 lenses as small as a pin's head, are all so accurately finished, 

 fitted, and adjusted, that in my opinion they are triumphs of 

 science and skill. 



In looking at an object with a high-power object-glass 

 made perfectly correct, both for chromatic and spherical 

 aberration, if you place over the object a piece of thin glass 

 with water or Canada balsam underneath, the corrections are 

 no longer strictly accurate, and it is necessary to make 

 different corrections in the object-glass. This is accomplished 

 by means of a screw in the collar which is made to turn round, 

 and the lenses can be made to approach to or recede one from 

 another. In this manner corrections can be made for the 

 difference in the conditions due to the glass cover. 



Many object-glasses are what are called dry lenses, that is 

 to say, there is air between the object-glass and the object. 

 In low powers this is almost invariably the case, but in some 

 of the high powers now made, advantage is taken of what is 

 called the immersion principle. In using such object-glasses a 

 small quantity of water is placed between the front lens and 

 the glass cover. One great advantage of this is that we get 

 a much greater amount of light. Another advantage is that 

 you can correct the lens more easily, and perhaps utilize a 

 wider beam of light. It would be tedious to enter into the 

 discussion that has taken place with reference to the difference 

 in the size and aperture in dry and immersion lenses. Many 

 most eminent authorities have differed exceedingly about this, 

 and a most angry discussion has taken place on the subject. 

 In any case, it appears that with immersion lenses we obtain a 

 greater amount of light, especially that of wide deviation from 

 the direct line of vision, which is of the greatest value in 

 defining minute structure. 



