42 THE MICROSCOPE. [March, 



lies the object in proportion. For example, suppose normal sight 

 is 10 in. ; then, if we use a lens which has a focal length of 5 in., 

 the eye is brought twice as near to the object, and consequently the 

 object will appear twice as large ; or if we use a lens 2\ in., then 

 the diameter will be magnified four times, and so on, up to \ an 

 inch or other higher power. Of course a high-power eyepiece 

 could be used on any object-glass. But there is this drawback : 

 in all object-glasses of high power, except those of the highest possi- 

 ble quality, there will be some little defect, and to use a high-power 

 eyepiece would be to magnify the image of the object, defects 

 and all. Thus, from the construction of the instruments, a much 

 higher power eyepiece can be used on the one than on the other. 



Eyepieces for the microscope are sometimes designated A, B, 

 C, D, etc. They consist of two lenses : a large one nearest the 

 object-glass, called the field lens, and a small one nearest the eye, 

 called the eye lens. 



A general rule for diameters is that the field lens should be 

 twice that of the eve lens ; thus, if the field lens is 1 in. in diameter, 

 the eye lens should be about J in. in diameter. The rule for local 

 length is that they should be as 2 to 1 ; thus, if the field lens is 

 1 J in., then the eye lens must be J in. These are placed with their 

 plane side towards the eye, and one-half of their combined focal 

 length apart ; thus i^ + | = 2 i ^- 2 = x 8' so tnat tne > r wou ld 

 be 1 J in. apart. A stop must be placed between these at the 

 focus of the eye lens, the opening the same as its diameter. 



To determine the focal length of the eyepiece, proceed as 

 follows : Multiply the focal length of the lenses into each other 

 and then by 2, and divide the product by the focal lengths when 

 added together ; thus 1} X i X 2= ij : ij -f- £ = 2 : 1 J -=- 2 

 = J, so that the eyepiece constructed by these lenses would be f-in. 

 focal length. 



The following are given by a manufacturing optician : 



1. Eye lens, 1 J ; field lens, 3 ; distance apart, i\. 



2. Eye lens, 1 ; field lens, 2 ; distance apart. \\. 



3. Eye lens, J; field lens, 1 \ ; distance apart, i\. 



4. Eye lens, \ ; field lens, 1 ; distance apart, J. 



In making eyepieces for the telescope, the relative focal length 

 of field lens to eve lens is 3 to 1. In mounting the lenses in their 

 cells, the plan already given should be followed. With these 

 principles, any clever worker in brass should find no difficulty in 

 making any required eyepiece. — Work, April. 1892. 



To Fix Paper on Glass or Metals. — The Revue Photo- 

 graphique gives the following, which is said to be excellent : Mix 

 30 gm. tragacanth and 120 gm. gum arabic, and dissolve in 500 

 gm. distilled water. Filter and add 2.5 thymol dissolved in 120 

 cem. glycerin. 



