F. F. Wright — New Petrograjphic Microscope. 413 



figures by the Lasaulx method without the use of the Bertrand 

 lens, chiefly because of the disturbing effects of diffraction from 

 the small apertures required and the distance of the aperture 

 from the eye of the observer. 



In designing this instrument, special attention has been paid 

 to adjustment facilities and arrangements by which adjustments 

 can be readily accomplished by the observer himself. In scien- 

 tific instruments in general, too much reliance is often placed 

 on the adjustment by the maker and the assumption " once in 

 adjustment always in adjustment" notwithstanding jars and 

 jolts of transportation, and the gradual relief of stress in any 

 complicated mechanical system. Fortunately, the principle, 

 which some manufacturers have adopted, of making all parts 

 rigid and eliminating adjustment facilities, cannot be carried 

 out in the construction of the microscope. In this instrument 

 the axis of both the upper tube and the condenser holder on the 

 microscope must coincide precisely with that of the revolving 

 stage, in order that in every position the optical system remain 

 centered. Since the ocular and the condenser remain auto- 

 matically centered with respect to the revolving stage, while 

 the objective changes its position slightly on each insertion, it 

 is necessary that centering screws (« l5 « 2 , fig. 1) be introduced 

 for the objective itself and the direction of motion of center- 

 ing should be parallel with the cross hairs of the ocular, as the 

 eye estimates much more readily coordinate directions than 

 diagonal directions. The principle sometimes adopted of 

 placing the centering screws with directions of movement 

 along the diagonals is therefore less favorable than along the 

 cross hairs, and places a burden on the microscopist which 

 might easily be avoided. The practice of placing the adjustment 

 screws on the revolving stage instead of above the objective is 

 wrong. The part of the optic system which is not in adjust- 

 ment is the objective, not the stage. The axis of revolution of 

 the stage should form the starting point for the adjustment of 

 the whole instrument and should always remain fixed in its 

 position. To this axis the ocular, condenser, and objective 

 should be adjusted, and since the ocular and condenser remain 

 practically stationary while the objectives are changed con- 

 stantly, the only logical point of adjustment, to obtain satis- 

 factory results, is above the objective. 



To summarize briefly, the most important changes intro- 

 duced on the present microscope are : (1) Both nicols revolve 

 simultaneously, the connection between the two being a rigid 

 bar, thus eliminating the errors due to lost motion in the gear- 

 wheels of the systems ordinarily employed for this purpose. 

 Since adopting the above device, the writer has learned that 

 the scheme of revolving the nicols by a rigid connecting bar 



