44 



LIGHTING AND FOCUSING [ CH. II 



fly's wing under the microscope ; put the 16 mm. {yi in.) objective 

 in position, and also the lowest ocular. Select the proper opening 

 in the diaphragm and light the object well with transmitted light 



(§ 73. 77)- 



Hold the head at about the level of the stage, look toward the 

 window, and between the object and the front of the objective ; with 

 the coarse adjustment lower the tube until the objective is within 

 about half a centimeter of the object. Then look into the micro- 

 scope and slowly elevate the tube with the coarse adjustment. The 

 image will appear dimly at first, but will become very distinct by 

 raising the tube still higher. If the tube is raised too high the 

 image will become indistinct, and finally disappear. It will again 

 appear if the tube is lowered the proper distance. 



When the microscope is well focused try both the concave and 

 the plane mirrors in various positions and note the effect. Put a 

 high ocular in place of the low one (§ 50). If the oculars are not 

 par-focal it will be necessary to lower the tube somewhat to get the 

 microscope in focus. 



Pull out the draw-tube 4 to 6 cm. , thus lengthening the body of 

 the microscope ; it will be found necessary to lower the tube of the 

 microscope somewhat. (For reason, see Fig. 65.) 



§ 83. Pushing in the Draw-Tube.— To push in the draw- 

 tube, grasp the large milled ring of the ocular with one hand, and 

 the milled head of the coarse adjustment with the other, and grad- 



write to Professor Abbe, and his reply, kindly given, is so clear and to the 

 point, and of such interest and value, that we take the liberty of publishing it 

 for the benefit of our readers." 



"Jena, June 25th, 1881. Dear Sir : The question which you ask admits of a 

 simple answer : In order to change the oculars of a microscope without chang- 

 ing the focus of the objective, neither the diaphragm nor the field lens must 

 come to the same place in the microscope tube, but the anterior (lower) focal 

 points of the ocular systems must do this. In the case of a Huygehenian 

 eyepiece, the said anterior focus 19 a virtual one situated above the field lens 

 at a place D*, which is more distant from the field lens .than the diaphragm D. 

 The level of D* is the place where the virtual image of the diaphragm appears 

 to an observer looking through the field lens. Rays which are required to 

 emerge from the eye lens as parallel rays (or nearly parallel) must of course 

 enter into the ocular converging to the point D*. Consequently if different 

 oculars are inserted successively in such a way that the point D* comes to the 

 same place of the tube always, the conjugate foci of object and image in the 

 objective remain unaltered." 



