CH. IX] MICRO- PROJECTION WITH OCULARS 231 



In using the ordinary oculars a small tube must be screwed into 

 the large microscope tube as for ordinary observation (fig. 147, 



197). 



Special oculars have been designed for projection. Some, like 

 those of Zeiss (fig. 128) give sharp brilliant images, but the field 

 is very small. Williams, Brown and Earle have a very large pro- 

 No. 2. 



No. 4. 



FIG. 128. PROJECTION OCULARS OF ZEISS. 

 (From Zeiss' Catalogue, No. jo). 



A section has been removed to show the construction. Both are of the 

 negative form. 



The eye lens is in a smaller tube with spiral movement to enable the operator 

 to focus the image of the diaphragm of the ocular sharply on the screen. 

 Below are shown in face view the upper ends of the oculars with their graduated 

 circles. By noting the position in any experiment it is easy to set the position 

 exactly the same if the experiment is to be repeated. 



No. 2, No. 4 These numbers indicate that the ocular magnifies the image 

 two or four times (see 391). 



jection ocular of the Huygenian form which magnifies about twice. 

 On account of the loss of light and the restriction of the field of 

 view, the writers of this book do not advocate the use of oculars for 

 ordinary micro-projection, but sec 401. 



357a. Designation of oculars. At the present time an ocular is usually 

 designated by the increase in magnification it gives a microscopic image when 

 the microscope is used in the ordinary way. For example, if the objective alone 

 would give an image 10 times as long as the object, then an ocular x 2 should 

 double that size, thus giving an image magnified 20 times, and an ocular x 4, 

 an image magnified 40 times and so on. 



