A sensitive diamond indenting tool was developed which gives indentations of accurately measurable length in the most resistant steels with loads of less than 1 kg. It was found that the use of these light loads permits extension of indentation tests to small specimens and to tests of brittle materials, such as glasses, which shatter under the heavy loads required by present indenters.The indenter is pyramidal in form, giving a diamonn-shaped (rhomb) indentation of which the diagonals have an approximate ratio of 7 to 1 instead of 1 to 1 for the square-based pyramid. Such indenters were duplicated to the required accuracy and showed no discernible wear with use.Elastic recovery of indentations with this indenter takes place in a transverse rather than a longitudinal direction, and, consequently, from the measured length of the long diagonal and the constants of the indenter, unrecovered dimensions of an indentation are obtained. Results of tests are expressed as indentation numbers which relate the applied load in kilograms to the unrecovered projected area in square millimeters.Recovered projected areas also, may be determined with an added measurement of the short diagonal. Since knowledge of both recovered and unrecovered dimensions may be obtained, the indenter offers possibilities for study of the fundamentals involved in indentation testing that are not afforded by ball, cone, and square-based pyramidal indenters, which, because of their symmetrical form, yield recovered dimensions only.The performance of seven indenters of different angular formation was investigated to determine their relative sensitivity and adaptability for use in different materials and to determine also the effect of load and of different dimensional bases of computation on indentation number. Consideration of the results led to the selection of an indenter which gives excellent performance in many materials. Comparison is made of indentation numbers given by this indenter and corresponding Brinell and Vickers numbers for the same specimens. CONTENTS Faga

I. Introduction 504 II. Apparatus and methods 1. The radiometer 2 Construction of the receivers 506 3. Thermopile and galvanometer 4. Water-cooled shutter and diaphragm 5. The radiator 6. The assembled apparatus 7. Method of making observations 8. Method of reduction of data 520 9. Corrections for diffuse reflection from the receiver 10. Accuracy attainable III. Experimental Data Receiver No. 1 Receiver No. 2 Receiver No. 3 Receiver No. 4 Receiver No. 5 Receiver No. 6 Receiver No. 7 Receiver No. 8 Receiver No. 9 Receiver No. 10 Receiver No. 11 Receiver No. 12 • Receiver No. 13 ,.

The compressibility of fused-quartz glass was determined with sufficient accuracy to establish the required corrections for change of length with change of pressure of the fused-quartz glass etalons used in a Fabry-Perot interferometer to evaluate the indices of refraction of air for converting standards of wave length in air to their value in vacuo. Compressibility determin-ations were made for the same changes of pressure that were used for index me surements; namely, from vacuum to atmospheric pressures.Determination of the compressibility of fused-quartz glass was made by interferometric compari son of the relative change in length of end gages of fus ed quartz and stainless steel.. Values for the compressibility of both fus ed quartz and stainless steel were also determined by a direct method which depended upon the relative change in length of a closed tube and a solid gage of the same material. By this method the compressibilities of fu sed quartz and of a stainless steel were determined independently , thcreby elim inating the existing uncertainty as t o the compressibility of steel at atmospheric pressure obtained by extrapolation from much higher pressures and eliminating, also, the necessity for comparison of materials with great difference in thermal expansivity.Relative values for fused quartz and stainless steel were also derived from comparative changes in length with pressure of tubes and gages of dissimilar materials.The measurem ents gave the foll owing values for longitudinal compressibility at 1 atmosphere pressure:For fu sed-q uartz glass 9.9X10-7 ±5X10-s per atmosphere. For a sample of stainless steel ::<3

The true contours, und istorted by gravitatio nal be nding, were determ ined for four 10 %-lIl ch-diameter stan dard optICal flats of fu sed quart z. The bending de fl ections of these flats \yere determined b~' a method base d upon the diffe rential be nd ing with thi c kn ess of t he flats. Bend ing deflect ion cu r ves of a fla t supported a' G t hree pOin ts equid istan t from the center o f the fla t and equ idistan t from each other were obt aine d. The locus o f the belld ing deflections at the center of a flat, s imilar!.v supported but with s upports at differe nt di, tances from the center , approx im ates a s traigh t line. This paper describes t he m ethod used to obtain the true conto ur'S and t he be nding de flection cu rves of the fla ts, and compares t he bending \'alues so determi ne d with t heo ret ically derived valLws.

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