The Use of the Microscope 37 



6. Clamp the microscope in position on the stage of the Ortho-Illumina- 

 tor. Those using a separate lamp will find it well worthwhile to provide 

 themselves with a stage to which both microscope and lamp can be 

 securely clamped. 



Everything is now lined up and it only remains to place a slide on the 

 stage, erect Kohler illumination in the manner described for the medical 

 microscope, and control intensity of illumination. On the Ortho-Illumina- 

 tor (Fig. 45) this is done by rotating into position a disc of the required 

 intensity. 



All this laborious business, which stems from having a centerable sub- 

 stage condenser, is necessitated by the fact that the lenses in even the 

 best turrets are not perfectly concentric. When, therefore, the X20, X40, 

 or X90 lens is rotated into place and the substage refocused, it will be 

 found that the image of the field iris is no longer perfectly central. It 

 should be brought back to center by moving the substage with its center- 

 ing screws. This movement will be of the slightest— if it is not, a com- 

 plaint to the manufacturer should at once be made— and the slight off- 

 centering of the field iris and light source will not be of importance. 

 Never recenter the image by shifting the field iris, as it is of much more 

 practical importance that the substage condenser be lined up with the 

 objective than that the field condenser be lined up with the substage 

 condenser. In fact, once the system has been clamped, no centering 

 screws except those of the substage condenser should ever be touched 

 unless the whole system is to be relined. 



The ideal system, of course, is one in which each objective can be 

 centered over the substage condenser after the latter has been lined up 

 with the field condenser and the light source. Such systems exist— Figs. 

 15, 16, and 17 could not have been taken without one— but they are 

 altogether beyond the scope of this book. It might be added that they 

 usually have to be lined up from scratch before each use, a procedure 

 that takes about 3 hr. 



Only a few operations with the research microscope remain to be 

 described. 



Setting Up an N.A. 1.4 Immersion System 



The reader who followed the discussion in the last chapter will realize 

 that this N.A. is only obtainable if the condenser, as well as the objective, 

 is working in oil. Therefore: 



1. Line everything up as previously described. 



2. Set the slide on the stage and get in Kohler illumination with a X40 

 objective. Find the area to be studied. 



