In order to do critical microscopic work, one 

 should have a box of neutral intensity filters, 

 graded 0.3, 0.6, 0.9, and 1.2. These numbers 

 represent the logarithms of the opacity ; each filter 

 has double the opacity of the one preceding it. 

 By placing falters, one can reduce the light until 

 there is no danger of eye injury. Place a pre- 

 pared slide on the stage of the microscope. Ad- 

 just the flat mirror so that, when the low-power 

 objective focuses on the specimen, the light falls 

 directly in the center of the microscope field. To 

 ascertain that it is centered, close the lamp dia- 

 phragm as far as it will go and then elevate the 

 microscope condenser until the opening of the 

 lamp diaphragm appears as a sharp ring. With 

 the flat glass mirror usually present on research 

 microscopes, there will be three circles instead of 

 one. Critical work can be carried out with the 

 glass mirror but for these studies on avian blood, 

 this mirror was replaced with a flat, front-sur- 

 faced mirror, which eliminated all images except 



one. 



After manipulating the mirror to bring the light 

 to the center of the field — or to what appears to be 

 the center — and after focusing the condenser to 

 the level of the specimen, check to see that the 

 circle of light really is in the center of the field. 

 This is the time to ask the question. Is the con- 

 denser perfectly in line with the optical axis of 

 the objective, tube, and eyepiece? To find the 

 answer, proceed as follows: 



Open the diaphragm on the lamp as far as it 

 will go, then close the microscope diaphragm — 

 all tiie way. Lift out the eyepiece and look 

 through the tube. There will be a circle of light 

 that does not fill the front lens of the objective. 

 Is this circle of light in the center? It is difficult 

 to tell, because moving the head makes the light 

 appear to move. Freedom from the uncertainty 

 can be gained by using a pinhole eyepiece, which 

 has a small hole in the exact center (but no lens) . 

 With this in place the position of the circle of 

 light can be determined easily and accurately. 

 If it is not in the center there are two or three 

 screws with which adjustment can be made. The 

 screws are on most microscopes. They are not 

 on some of the less expensive student instruments. 

 If an adjustment is necessary to bring the im- 

 age of light to the center of the front lens, make 

 it, then return the regular eyepiece to the tube. 

 Now open the microscope diaphragm and close 

 the lamp diaphragm. Is the image still in the 



center of the field. If it is not, readjust the mir- 

 ror, look through the open tube of the microscope, 

 and check again with the pinhole eyepiece. 



Wiien, finally, the condenser seems to be in per- 

 fect alignment, it can be checked by moving the 

 tube up and down. The circle of light will en- 

 large above and below the focal point. If these 

 two light cones remain on the same axis within the 

 tube the task has been completed. But if the 

 light beam shifts to one side of the tube axis as 

 the tube is moved up and down, the adjustment 

 procedure should be repeated. Books by Bell- 

 ing (1930) and by Beck (1938) should be con- 

 sulted for further refinements on testing the align- 

 ment of the condenser. 



After the tests on alignment have been com- 

 pleted, put in the eyepiece and open the lamp 

 diaphragm all the way. It probably has already 

 been noted that the field of light from the lamp 

 illuminates only a small circle in the center of 

 the low-power field and that the marginal half 

 or two-thirds of the field is dark. The tendency 

 of many is to drop the condenser until the field 

 is fully lighted. Doing this is unobjectionable 

 if you wish only to examine large masses in the 

 slide, but it is objectionable if you are interested 

 in details. The correct procedure for filling the 

 low-power field with light will be given after ad- 

 justments relating to the use of the oil immersion 

 lens have been described. 



Starting with the microscope condenser ele- 

 vated so that the edge of the lamp diaphragm is 

 in sharp focus and wide open, shift to the ob- 

 jective of next highest magnification and note 

 whether the field of light is still in the center. 

 Do the same for the other objectives. If all the 

 lenses do not show the circle of light in the center 

 of the field, at least one is not par centered. This 

 adjustment should be made at the factory or with 

 the help of a competent representative of the com- 

 pany. If most of the microscope work can be 

 done with only one lens, all adjustments can be 

 made to fit the particular lens. (Most of the 

 work reported in this Atlas was done with the oil 

 inmiersion lens.) All the lenses screwed into 

 one nosepiece should be par focal: as each is 

 moved into position, only a slight movement of the 

 fine-adjustment screen should be necessary to 

 bring it into focus. Shifting lenses in this way 

 is recommended for the "dry" lenses Imt certainly 

 it cannot be reconunended as standard procedure 

 for the oil immersion lenses. How to bring the 



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