534 G. H. BEAVEN, E. R. HOLIDAY, AND E. A. JOHNSON 



oriented by flow) in which the principal absorption directions are in all 

 possible directions with respect to the electric vector of the (nominally) 

 unpolarized incident radiation. In order to measure the principal absorptiv- 

 ities it is necessary to use crystals or oriented specimens and plane-polarized 

 radiation. Ideally, the arrangement of the molecules in a crystal with re- 

 spect to the crystallographic axes should be known from an X-ray structure 

 determination, but even if this information is lacking the dichroism alone 

 can be used to obtain information about the probable orientation of the 

 absorbing groups within the crystal. 



In practice absorption measurements on crystals or oriented material 

 usually entail the use of very small specimens. This limitation arises partly 

 because of the difficulty in preparing large crystals or specimens (or the 

 inherent size of specimens of biological origin), and partly because of the 

 high absorbances that would be encountered with thick samples. For this 

 reason, it is necessary to use some form of fully achromatic reflecting micro- 

 scope in conjunction with the spectrophotometric equipment. With regard 

 to crystals, the number of compounds in which the arrangement of the 

 molecules within the crystal lattice is known from X-ray structure data to 

 be favorable for the study of pleochroism is very small. The total volume 

 of work in the field is therefore limited, though the introduction of various 

 designs of reflecting microscope has led to increased activity. 



Seeds^"' has reviewed the polarized ultraviolet microspectrography of 

 crystals and oriented systems of biological interest, and this work should 

 be consulted for experimental details. General microspectrophotometry 

 has been dealt with in considerable detail by Seeds, Wilkins, Barer, Davies, 

 Mellors, Walker, Commoner, and other contributors to a recent sym- 

 posium^"* which contains much information on reflecting optics and micro- 

 spectrophotometric systems, together with typical results on a wide variety 

 of biological material. 



2. Form Dichroism 



Form anisotropy, both birefringence and dichroism, is encountered in 

 composite systems containing oriented assemblages of macromolecules, 

 even if the latter are themselves isotropic, when the systems contain amor- 

 phous regions between the ordered ones. The fonn anisotropy is additional 

 to any intrinsic anisotropy that the oriented macromolecules may exhibit 

 and can therefore make an important contribution to the total observed 

 anisotropy of a system in which the conditions for form anisotropy are 

 satisfied. The two models which have been treated theoretically by Wie- 



'<" W. E. Seeds, Polarized ultraviolet microspectrography and molecular structure, 



Progr. Biophys. and Biophys. Chem. 3, 27-46 (1953). 

 1"* Spectroscopy and molecular structure and optical methods of investigating cell 



structure, Discussions Faraday Soc. No. 9 (1950). 



