Till'; MOLD.WITKS 28^-) 



tu^urapliic plate. The optical effects resulting I' rum strain were first 

 studied in detail l)y Brewster in 1(SL4, at a time wlien only the simplest 

 of optical apparatus was available and but little was known of double 

 refraction. JN'otwitb standing tbis, Brewster deduced fi'om a series of in- 

 genious experijucnts many of tbr fundamental laws of the optical be- 

 liaxior of glass straiiird ritliei- mccbanically by ditt'erential pressure or 

 tension or as a result of non-uniform heating or cooling. Brewster found 

 that a plate of glass under load is bii'efrat'ting : tliat the optical effect 

 ])rodiiced is sensibly ])roportional to the intensity of the strain; that a 

 plate of glass under differential compression beha^■es optically as a uni- 

 axial negati^•e crystal with its principal axis in the direction of the acting 

 load, while under differential tension it acts as an optically ])ositi\'e uni- 

 axial crystal ; that in a glass plate cooled quickly from a high temperature 

 a permanent strain is imparted which is at maximum intensity next to 

 the outer surfaces (zone of compression), and which, decreasing toward 

 the center, reaches a neutral band and passes then into a zone of tension 

 in the central part of the plate: that compression produces retardation, 

 while dilatation causes acceleration of the transmitted light waves. 



Since Brewster's time improvements have been made in the methods 

 of observing and measuring strain birefringence, but these rt'finements 

 are not required in the present problem. To study the distribution of 

 strain in an irregular glass fragment, the only a])])aratus required is two 

 crossed nicols and a sensitive tint plate. This is easily obtained by re- 

 moving condenser, objectives, and eyepiece from the microscope and ob- 

 serving the fragment immersed in a liquid of the same refractive index. 

 l''or this ])iirpose a small crystallizing dish or breaker is Mell suited as a 

 container, and benzol, with refractive index approximately 1.50, as an 

 iinmci-sion liipiid. Tlie purpose of the refracti\'e litpiid is io o\"ei'conie 

 the annoving sui'facc i-ellections from the glass surface, whicli tend Io 

 distui'li and to mask the intei'i'erence phenomena resulting fi-oin sti'ain. 



Relufiiing now to the moldavites, we have three possibilities to con- 

 sider : 



( 1 ) The molda\ ites are etched fragments of a large mass of slowl\- 

 cooled obsidian. In this case, as we have seen above, little, if anv, sti'ain 

 is present. Between ei'ossed incols the fragment is practically isotroi)ic. 



(2) The moldavites are volcanic ejectaraenta which weic oi-iginally 

 molten, but were chilled rapidly during contact with the air. In this 

 case they should show a considerable amount of sti'ain, with an outer 

 zone of compression, an intei'mediate zone of no strain, and a central 

 region under dilatational strain. 



