BASIC DESIGN, OPERATION AND USE 



The eyepieces are normally three-element 

 construction, with external focal plane. They 

 have about 25% larger fields than conven- 

 tional microscope eyepieces, and are often 

 termed "widefield" eyepieces. 



The objectives are each achromatic dou- 

 blets. To change power, another objective 

 pair of different focal length must be sub- 

 stituted. This is normally done by mounting 

 several objective pairs on a sliding or re- 

 volving nosepiece. 



Instead of changing objectives, it is pos- 

 sible also to change lenses within the body 

 of the microscope to achieve a power change. 

 One such system employs parallel light 

 beams beyond the objectives, into which 

 small Galilean telescopes are inserted in 

 either normal or reversed direction. A 2X 

 Galilean system, so used, multiplies the 

 power by either 2 or 3^^, depending on its 

 orientation. A second pair of Galilean tele- 

 scopes may similarly give 3 X and V3 X . 



A more elaborate power changing system 

 is to make the objectives continuously vari- 

 able in power. Such systems are known as 

 "zoom" lenses. With these, the field of view 

 is never lost during power change, so that 

 there are no blind spots in converting gradu- 

 ally from low to high power. This is an im- 

 portant point, particularly in teaching, where 

 the student may more easily grasp the rela- 

 tionship of the part to the whole in a bio- 

 logical specimen. 



Figure 2 shows a side view of the optical 

 system of a zooming stereomicroscope. The 

 complete system, of course being actually 

 two such systems, arranged to aim at a 

 common object point, as in Figure 1. The 

 zooming lens system comprises three pairs 

 of doublet lenses, the upper two pairs being 

 accurately cam-driven to hold the image 

 in focus as the power is changed. 



Figure 3 shows a front view of the cam 

 system used to drive the movable lenses. 

 The power change knob has a gear on its 

 mounting shaft which drives two identical 

 gears on two identical cam shafts. Thus the 



/vEPicce 



PIECE FOCAL 



Fig. 2. The optical system of a continuously 

 variable power stereomicroscope. The movable 

 lenses are shown in the two limiting positions of 

 their cam driven motion. 



INTEROCULAR 

 ADJUSTMENT 

 AXLE 



INCLINED PORRO 

 MIRROR SYSTEM 

 (4 MIRRORS) 



CAM DRIVEN 

 UPPER LENS 

 MOUNT 



CAM DRIVEN 

 LOWER LENS 

 MOUNT 



MOUNT FOR 

 STATIONARY LENS 



Fig. 3. The mechanical structure of the micro- 

 scope shown in Figure 2. A single knob drives both 

 cams, thereby synchronizing the power change in 

 both left and right sides of the microscope. 



cam-driven lenses move in accurately syn- 

 chronized motions so that left and right 



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