CH. I] 



MICROSCOPE AND ACCESSORIES 



Hold the simple microscope directly toward the sun and move it 

 away from and toward a piece of printed paper until the smallest 

 bright point on the paper is obtained. This is the burning point 

 or focus, and as the rays of the sun are nearly parallel, the burning 

 point represents approximately the principal focus (Fig. n). 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 

 centimeter 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 (§53). 



After getting as clear an image as possible with a simple micro- 

 scope, 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 posi- 

 tion, the largest field may be seen. This position corresponds to the 

 eye-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. 



Fig. 17. 

 Tripod Magnifier. 



Fig. 19, Lens-holder ( The Bausch & 

 Lomb Optical Co. ) 



Fig. 18. The Hastings Aplan- 

 atic Triplet. ( The Bausch & Lomb 

 Optical Co. ) 



