106 INSTRUMENTATION 



definitions a, b, c, and d. In terms of this notation, phase objectives 

 of the type TA — 0.25X produce dark contrast in most biological applica- 

 tions, and phase objectives of the type TA+0.25X generally produce 

 bright contrast. The value of T required to produce a given amount of 

 contrast depends strongly on the dimensions of the conjugate area. The 

 omission of any designation of pi and p2 is a shortcoming when a particu- 

 lar observation is being described, and a more inclusive nomenclature 

 would be very useful. 



3. ALIGNMENT OF THE PHASE MICROSCOPE 



The most successful use of either the ordinary or the phase microscope 

 demands correct alignment of the optical system and the source of light. 

 The phase microscope needs only a few relatively simple adjustments in 

 addition to those re(iuired for the correct alignment of the ordinary 

 microscope. As in bright held microscopy with central conical il- 

 lumination, optimum performance is obtained w^hen the light source is 

 centered with respect to the substage mirror; when the center of the 

 substage mirror, the optical axis of the substage condenser, the optical 

 axis of the objective, and that of the ocular are in line; and when the 

 required field and aperture of the objective are uniformly illuminated. 

 Phase microscopy imposes the additional conditions that the plane of the 

 image of the opening in the condenser diaphragm shall coincide with the 

 plane of the conjugate area of the diffraction plate and that the image 

 of this opening shall fall within the conjugate area. Definition and 

 contrast in the image are sensitive to this alignment of the opening in the 

 condenser diaphragm with respect to the conjugate area of the diffraction 

 plate. Passage of part of the undeviated light through the comple- 

 mentary area generally causes a rapid loss of contrast in the image. It 

 was pointed out in Section 1 that parallax between the image of the 

 opening in the condenser diaphragm and the conjugate area of the 

 diffraction plate also can cause loss of contrast. The diffraction plate 

 should be centered on the optical axis of the objective by the 

 maker. 



No two manufacturers supply an identical set of phase accessories, 

 just as no two manufacturers make identical microscope stands and 

 identical sets of optics. However, as with the ordinary microscope, each 

 maker's equipment performs similar functions, and a recommended 

 procedure for aligning the phase microscope may be described in terms of 

 the equivalence of the various parts. The diffraction plate may be 

 separable from the objective or it may be fixed in the objective. There 

 is a mount into which a condenser is fitted and which also carries the 

 diaphragm to be used with a given phase objective. The substage 



