USE AND MANIPULATION OF MICKOSCOPE 165 



to project the image on to this opaque screen, and exactly 

 centre the image of the illuminant to the centre of the field. 

 In fact, given a sufficiently powerful source of light, it is 

 by far the best method to make all the adjustments, centreing 

 the condenser and the illuminant, getting the correct position 

 of the object in the field, and closing the iris-diaphragm to 

 the required amount, by observing the projected image. An 

 illuminant of great power is not necessary for this purpose. 

 Among the electrical ones, a one-ampere Nernst lamp on a 

 240-volt circuit gives quite sufficient light to make the necessary 

 adjustments even with oil-immersion objectives. A small arc 

 lamp such as the Leitz form taking four amperes, will enable 

 adjustments to be effected if an object is being magnified as 

 much as two thousand diameters. It is obviously easier to 

 exactly centre the optical components and the illuminant with 

 a large image projected to a distance, and with the operator 

 free to adjust any of them with facility, than to do it while 

 observing the image in the microscope. 



To prevent confusion it may be wise to recapitulate here 

 the exact order in which the operations described should be 

 performed : 



1. Set up and centre the object with the objective with 

 which the photograph is to be taken. 



2. Eemove this objective, if it is a high-power one, and 

 substitute for it a low or medium power. 



3. Close the iris -diaphragm in the sub-stage condenser as 

 far as possible. 



4. Back down the sub-stage until a clearly defined image 

 of the opening in the iris- diaphragm is seen in the field. 



5. Centre this image of the iris- diaphragm until it is in the 

 centre of the field ; or alternatively, where a change of objec- 

 tive is necessary, until the image is exactly projected over the 

 point of intersection of the diamond lines on the slide already 

 described. 



6. Back up the condenser until an image of the illuminant 

 is exactly focassed in the field of view, the object itself also 

 being in correct focus. 



7. Centre the image of the illuminant by moving the source 

 of light so that it falls exactly across the microscopic image. 



In cases where the illuminant is of large area, and where 



