77 



multiplies the virtual image by the extent of its 

 power. From this it can be easily computed that 

 with a 1 inch objective used with a tube length of 

 10 inches and a 1 inch eyepiece, a magnifying 

 power of 10X10 = 100 will be obtained; or the 

 same combination with a tube length of 5 inches 

 will give one-half this power or 50. 



Objectives. These are divided into two classes, 

 dry and immersion. In the dry there is no inter- 

 vening medium other than air between the bottom 

 lens of the objective and the top of the cover-glass. 

 In the immersion a liquid fills up this space. From 

 this fact it is easily seen that liquid can only be used 

 in objectives which are quite close to the cover and 

 therefore short focus or high power and so on the 

 other hand can objectives of long focus or low 

 power not be immersion. We will see later that 

 the purpose of the immersion is to obtain higher 

 optical results, is in fact a necessary condition, and 

 an objective which is constructed as a dry one 

 cannot be used as an immersion, and vice versa. 

 Although there have been objectives constructed 

 which can be used both as immersion and dry, 

 they have gone into disuse as they must suffer 

 when used in one or the other direction or in both. 



Water was for many years used as immersion 

 fluid, but cedar oil was discovered and as by means 

 of this better results are obtained, it has largely 

 taken its place. Its optical properties are almost 



