ALIGNMENT OF THE PHASE MICROSCOPE 107 



condenser mount may be a single-unit type such that a cell containing a 

 diaphragm is screwed in or out of position or such that a plate provided 

 with a diaphragm can be slid beneath the substage condenser and locked 

 into the correct plane. Means are provided for centering the condenser 

 diaphragm with respect to the conjugate area of the diffraction plate. 

 (The diaphragm containing the opening required by the diffraction plate 

 will be referred to as the condenser diaphragm, and the iris diaphragm 

 which controls the numerical aperture of the condenser and which is 

 usually found at or near the first focal plane of the substage condenser 

 in the ordinary microscope will be designated as the iris diaphragm of 

 the condenser.) Sometimes means are provided for centering the sub- 

 stage condenser with respect to the objective. 



An auxiliary optical system is supplied in order to enable the user to 

 view the plane of the diff'raction plate at sufficient magnification to 

 facilitate the centering of the condenser diaphragm with respect to the 

 conjugate area of the diffraction plate. Often this is a separate unit 

 which fits into the body tube of the microscope stand in place of an 

 eyepiece. This unit, which is a microscope ha\'ing a long working 

 distance and which is generally called the centering telescope, is usually 

 adjustable in length so that the working distance and magnification can 

 be changed. Sometimes, as on the stand manufactured by the firm of 

 Galileo, the eyepiece of the phase microscope remains in position and 

 another lens element is moved into place below the eyepiece so that the 

 combination forms the centering telescope. The objective lens of this 

 centering telescope is removed from the path of light when the eyepiece 

 is again to serve as the ocular of the phase microscope. 



The elementary theory developed in Chapter II specified Kohler 

 illumination. Section 10 of Chapter VH discusses the fact that under 

 certain conditions of critical illumination the appearance of the image 

 of the object specimen in the phase microscope is similar to the appear- 

 ance of the image with Kohler illumination. The conditions necessary 

 in order that critical illumination cause the energy density in the image 

 to become similar to the energy density produced with Kohler illumina- 

 tion are that the source of illumination be broad and fairly uniform, that 

 a large field of view be illuminated, and that the speed of both the lamp 

 and the substage condenser be high. These conditions are usually 

 satisfied in the normal use of the microscope with critical illumination. 

 When the method of alignment recommended by a manufacturer 

 follows the procedure for Kohler illumination, the user is assured that, 

 if he so adjusts his equipment, not only is the field of view as uniformly 

 illuminated as is feasible but also that the image of the opening in the 

 condenser diaphragm is formed with the correct magnification and as 



