196 LECTURES TO SCIENCE TEACHERS. 



microscopes. Success depends almost entirely on the property 

 I have described, and on compounding together flint and 

 crown glass, in such a manner that you can take advantage 

 of a considerable amount of refractive power, but almost 

 entirely obliterate the dispersive power. This may be done 

 by combining a double convex lens of crown glass with a 

 plano-concave lens of flint glass, and when the surfaces have 

 the proper curves, and the lenses are of the proper thickness, 

 the dispersive power of the crown glass may be counteracted, 

 whilst its refractive power is only partially reduced. We 

 can thus construct lenses of moderately short focus length, 

 in which the images of different rays are brought to the same 

 focus, and thus can obtain by this kind of combination 

 simple achromatic lenses. 



I will now proceed to the consideration of compound 

 microscopes, because these are in the present state of science 

 far the most important. The characteristic principle of these 

 consists in their having an object-glass placed at the lower 

 end of the body of the instrument, which forms at the upper 

 end an image of any object on the stage. This image is itself 

 considerably larger than the object, the diameter being as 

 many times greater as the length of the tube from the object- 

 glass to the image is greater than the distance from the object- 

 glass to the object on the stage. The image thus formed is 

 inverted both vertically and laterally, and Avould appear per- 

 fectly distinct if it were thrown upon a white surface and 

 examined in front, or on ground glass and seen from behind. 

 If no such screen be placed to receive the image, it is still, as 

 it were, formed in free space and is capable of being again mag- 

 nified by another lens, or system of lenses. The principle of 

 compound microscopes consists in their forming an enlarged 

 inverted image by means of an object-glass, and in making it 

 visible when near to the eye, or, so to speak, magnifying it 

 with an eye-piece. 



In the very earliest compound microscope this eye-piece 

 consisted of a plano-convex or double-convex lens, but it was 

 soon found that a combination of two plano-convex lenses gave 

 a far better result. Modern eye-pieces are of two kinds. The 

 Huyghean eye-piece consists of two plano-convex lenses the 

 tipper one called the field-lens and the lower the eye-lens, 

 with a diaphragm between them at the focal point of the eye- 

 lens. The eye-lens is principally concerned in magnifying 





