CH. I] 



MICROSCOPE AND ACCESSORIES. 



Fig. 17. 

 Tripod Magnifier. 



point represents approximately the principal focus (Figs. 10, 11). With- 

 out changing the position of the paper or the magnifier, look into the 

 magnifier and note that the letters are very in- 

 distinct or invisible. Move the magnifier a cen- 

 timeter or two farther from the paper and no 

 image can be seen. Now move the magnifier 

 closer to the paper, that is, so that it is less than 

 the focal distance from the paper, and the letters 

 will appear distinct. This shows that in order 

 to see a distinct image with a simple microscope, 

 the object must always be nearer to it than its 

 principal focal point. Or, in other words, the 

 object must be within the principal focus. Com- 

 pare (§ 49.). 



After getting as clear an image as possible with a simple microscope, 

 do not change the position of the microscope but move the eye nearer 

 and farther from it, and note that when the eye is in one position, the 

 largest field may be seen. This position corresponds to the ej^e-point 

 (Fig. 30) of an ocular, and is the point at which the largest number of 

 rays from the microscope enter the eye. Note that the image appears 

 on the same side of the magnifier as the object. 



Simple microscopes are 

 very convenient when 

 only a small magnifica- 

 tion (Ch. IV) is desired, 

 as for dissecting. Achro- 

 matic triplets are excel- 

 lent and convenient for 



Fig. 18. Achromatic Triplet for the pocket. As shown in the left hand figure it 

 is composed of three lenses, one of crown and iivo of 'flint glass. The whole is 

 protected by a metal covering when not hi use. (Bausch & Lomb Opt. Co.) 



the pocket (Fig. 18). For use in conjunction with a compound micro- 

 scope, the tripod magnifier (Fig. 17) is one of the best forms. For 

 many purposes a special mechanical mounting like that of Figs. 19, 20, 

 is to be preferred. 



COMPOUND MICROSCOPE. 



2 10. A Compound Microscope. — This enables one to see an enlarged, inverted 

 image. It always consists of two optical parts — an objective, to produce an en- 

 larged, inverted, real image of the object, and an ocular acting in general like a 

 simple microscope to magnify this real image (Fig. 21). There is also usually 



