WRIGHT: INTERFERENCE FIGURES IN OBJECTIVES 303 



This distribution of colors in the interference figure on inser- 

 tion of the sensitive tint plate is precisely that observed on a 

 weakl}^ birefracting, optically positive, uniaxial mineral under 

 similar conditions and would lead one to infer that the objective 

 is uniaxial and optically positive in character. All high power 

 objectives show this behavior and it is of interest to inquire 

 into the cause of the conversion of an isotrophic substance, like 

 glass, into an apparently uniaxial, optically positive substance. 

 The problem can be solved both by experiment and by theoretical 

 computation. 



Experiment 1. Remove from the petrographic microscope the 

 condenser, objective and eye piece and place a sensitive tint plate 

 between the crossed nicols so that one of its vibration directions in- 

 cludes only a small angle (+2°) with the principal plane of the polar- 

 izer. Turn the analyzer through a small angle (+4°) clockwise and 

 note that the interference color changes from the sensitive tint to a 

 greenish hue; if now the analyzer be rotated counterclockwise to 

 — 2° the hue changes from green through the sensitive purple tint 

 to a magenta. Further rotation of either the sensitive tint plate 

 or the analyzer through large angles increases the illumination of the 

 field to such an extent that the faint differences in color hue are prac- 

 tically masked and cannot be readily detected. 



Experiment 2. Prepare fine glass beads, 0.1 to 0.5 mm. in diame- 

 ter, by fusing the end of a fine glass thread in a small Bunsen flame, 

 observe the beads under a low power objective between crossed nicols 

 and note that interference phenomena, similar to those recorded above 

 in the high power objective, appear when the sensitive tint plate is 

 inserted in the different positions, especially when its vibration direc- 

 tions include only a small angle with the principal nicol planes. The 

 same statement holds true for small air bubbles in Canada balsam or 

 glycerine. 



Experiment 3. Observe that on the outer margins of the glass 

 beads of experiment 2 the color phenomena are reversed, a nar- 

 row blue-green fringe appears as a frame about the orange yellow 

 quadrants and an orange yellow line on the periphery of the blue 

 green quadrants. The same phenomena are clearly shown on the air 

 bubbles in Canada balsam, and glycerine. If the optical character 

 were judged by these outlying colors alone, it would be uniaxial negative. 

 The air bubbles and glass beads appear, in short, to be optically posi- 

 tive in the center and optically negative at the margins. 



Experiment 4. Place small drops (0.5 to 1 mm. in diameter) of 

 mercury on an object glass under a low power objective. Raise the 

 condenser, so that steeply inclined rays are reflected from the equa- 

 torial zone of the upper half of the mercury drops. Observe that 

 only the outer margin of each drop is illuminated and that the in- 



