Techniques of determining spot size and sensitivity of the instrument will be developed. Spot sizes of the order of 5 microns have been obtained with the possibility remaining for improvement- Preliminary sensitivity measurements indicate that micro-microgram quantities of some elements are sufficient to be detected. Difficulties involved in finding desired target areas will be discussed. The inhomogeneities of standard samples used for conventional fluorescense X-ray quantitative analyses precludes their use for microemission techniques at least in some cases. Possible ways of determining homogeneity will be discussed. Analysis which can be performed to advantage may be classed into several categories.

• 1. The changes in composition occurring in grain boundaries can be detected and compared with the base material.

• 2. An inclusion, in a metallic material can be brought under the electron probe and analyzed independently of the base material.

• 3. Changes in composition across a boundary between solid phases can be measured.

• 4. Diffusion coefficients between solid phases may be determined.

• 5. Thickness of thin films and variations in film thickness can be measured.

Examples of the instrument's performance will be given in some of these categories.