Ultrasoft x-ray photons in the 24044 A range have significant advantages over electrons in the imaging of relatively thick, wet, unstained biological specimens. Considerations of specimen damage alone have indicated a resolution limit on the order of 100 A for specimens up to several micrometers thick. Although this resolution has been approached in contact microscopy of thin specimens, the technique is cumbersome and may be inappropriate for high resolution study of thicker specimens. Among the alternatives, the scanning technique has serveral advantages over the use of post-specimen optics in the formation of high resolution images. The radiation exposure can be lowered by the elimination of the large losses often encountered in magnifying optics and detectors. In addition, alignment and registration procedures can be more easily implemented. Finally, the specimen can be maintained in an open environment, since the path length between the vacuum window and the detector can be made short enough to allow appreciable transmission of the radiation. Researchers have embarked on a program to build a high resolution scanning transmission microscope. Our microscope will focus ultrasoft synchrotron radiation down to a small spot whose size is, essentially, the resolution of the microscope. The sample will be scanned throughmore » the spot in air and the transmitted radiation will be detected by a flow proportional counter and used to modulate the intensity of the displayed image. Information about the elemental composition of the sample can be obtained either by comparing two images taken at wavelengths stradding an absorption edge or by fluorescence microscopy using harder radiation and energy-dispersive detectors.« less