150 C. Barus — Colors of cloudy condensation. 



Steel knife-edges have been very generally used in pendu- 

 lum work but they are inferior to those made of agate. The 

 brittleness of the latter is rather an advantage than otherwise. 

 When an agate knife-edge receives a blow sufficient to injure 

 it, a piece is chipped out, leaving the remaining portion of the 

 edge clean and perfect as before and the only effect on the 

 vibratory period is that due to the removal of the matter lost. 

 Under the same circumstances an edge of steel will be flat- 

 tened or distorted and while there may be no loss of matter 

 the edge may be made so imperfect that the period will be 

 decidedly affected. 



The grinding of an agate edge demands much more labor 

 and skill than is required in the preparation of one of steel 

 but it is well worth the additional cost. 



It has been found that a knife-edge, even if its mass is con- 

 siderable, is very susceptible to such distortion of figure as 

 will render it imperfect. To avoid this it has been found de- 

 sirable to insert the edge in a heavy tablet of brass and grind 

 it in situ. The tablet is provided with three feet with hemi- 

 spherical ends, resting respectively in a conical hole, a Y 

 groove and on a plane. In this way the edge is subjected to 

 no strain after it is ground. 



Art. XIX. — 1 Preliminary note on the colors of cloudy con- 

 densation i by C. Barus. 



By allowing saturated steam to pass suddenly from a higher 

 to a lower temperature (jet) in uniformly temperatured, uni- 

 formly dusty air the following succession of colors is seen by 

 transmitted white light, if the difference of temperature in 

 question continually increases : Faint green, faint blue, pale 

 violet, pale violet-purple, pale purple, muddy brown-orange, 

 straw-yellow, greenish yellow ; green, blue-green, gray-blue, 

 intense blue, indigo, intense dark violet, black (opaque) ; 

 intense brown, intense orange, yellow, white. 



Seen by reflected white light, the same mass of steam is 

 always dull neutral white. 



If tbe colors enumerated be taken in the inverse order'be- 

 ginning with white, they are absolutely identical with the 

 interference colors of thin plates (Newton's rings) of the first 

 and second order, seen by transmitted white light under 

 normal incidence. Thus it is worth inquiring whether small 

 globules of water, when white light is normally transmitted, 

 affect it like thin plates. For a given homogeneous color if / 



