THE MICROSCOPE. 



ing microscopes. 8. Conditions of distinct vision in the microscope. 

 9. Effects of different magnifying powers. 10. Distinctness of deline- 

 ation necessary. 11. Hence aberration must be effaced. 12. Achro- 

 matic object-lenses. 13. Sufficient illumination necessary. 14. 

 Effects of angular aperture. 15. Experiments of Dr. Groring. 16. 

 Method of determining the angular aperture. 17. Mutual chromatic 

 and spherical correction of the lenses. 



1. THE microscope is an optical instrument by means of which 

 we are enabled to see objects or the parts of objects too minute to- 

 be seen distinctly with the naked eye, the name being taken from 

 two Greek words, tuKpbv (mikron), a little thing, and ovceTreo? 

 (skopeo), I look at. 



2. In a certain sense, all magnifying-glasses may be regarded as- 

 microscopes, but the slightly convex lenses, used by weak-sighted 

 persons, cannot with any propriety be so denominated. The 

 higher magnifying lenses, however, used by watchmakers, 

 jewellers, miniature painters, and others, may with less impro- 

 priety receive the name ; and the small lenses of short focus and 

 high power described in our Tract on " Magnifying Glasses," 

 especially when they have the form of doublets, and are mounted 

 to serve anatomical purposes, and for microscopic delineations, are 

 generally designated simple microscopes. Since, however, they 

 differ in no respect in their optical principle from common magni- 

 fiers, we have considered it more convenient to explain them 

 under that head, limiting therefore the subject of the present 

 Tract to those optical combinations which are generally called 



COMPOUND MICKOSCOPES. 



3. Such an instrument, in its most simple fornij consists of a 

 magnifying lens or combination of lenses, by means of which an 

 enlarged optical image of a minute object is produced, and a 

 magnifying lens, or combination of lenses, by which such image is 

 viewed, as an object would be by a simple microscope. 



4. The former is called the OBJECT-GLASS, or OBJECTIVE, since 

 it is always directed immediately to the object, which is placed 

 very near to it ; and the latter the EYE-GLASS, or EYE-PIECE, inas- 

 much as the eye of the observer is applied to it, to view the 

 magnified image of the object. 



5. Such a combination will be more clearly understood by 

 reference to fig. 1, where o is the object, LL the object-glass, and 

 E E the eye-glass. 



The object-glass, L L, is a lens of very short focal length, and 

 the object o is placed in its axis, a very little beyond its focus. 

 According to what has been explained in our Tract upon " Optical 

 Images," 31 et seq. an image o o, of o, will be produced at a dis- 

 tance from the object-glass L L, much greater than the distance of 



