LESSONS IN PHYSICS. 



81 



be placed beyond the principal focus, and less than twice the dis- 

 tance of the principal focus, the image will be inverted and larger 

 than the object. (See Fig. 16.) Again, if it is twice the distance 

 of the principal focus the image will be inverted, but of the same 

 size. (See Fig. 17.) If the 

 object be placed more than 

 twice the distance of the 

 principal focus the image will 

 be inverted and smaller. 



For the image in the case of the double concave lenses (see Fig. 

 18.) MN represents a double concave lens ; suppose the object at 

 AB, the rays of light are made to diverge in passing through 

 the lens, and if received by the eye at E, they appear to come 

 from a b, and the image is on the same side of the lens as the ob- 

 ject, erect and smaller. 



The compound micro- 

 scope consists of two 

 double convex lenses, the 

 smaller called the object 



glass and the larger the 



' ' s ' ' 



eye-piece. If the object is placed just beyond the focus of the 

 object glass, an image will be formed on the other side which is 

 inverted and larger than the object; the eye-piece is so placed 

 that the image formed by the object glass falls just within its 

 focus, so that it is again enlarged by the eye-piece, as in Fig. 15. 



The refracting tele- 

 scope consists of two 

 double convex lenses, 

 the larger called the 

 object glass, the 

 smaller the eye-piece. 



Fie. 1 8. 



The object is so far beyond the principal focus of the object glass 

 that the image is inverted and smaller, and the eye-piece is ad- 

 justed so that the image formed by the object glass falls just 

 within its principal focus, and is magnified by the eye-piece. In 

 a microscope an object is magnified both by the object glass and 



L. S. <j 



