MA.Y 4, 1906.] 



SCIENCE. 



707 



steplike double or single wedge in cross sec- 

 tion, the appearance is very striking, each 

 thickness glowing with its particular color. 



To produce a continuous wedge effect, the 

 width of the strips should be gradually de- 

 creased to the center of the field. Since stress 

 is greatest where the cross-section is least, the 

 higher orders of colors appear nearer the 

 center. 



5. Compensation Effects. — If a plate quartz, 

 cut parallel to the axis and showing the warm 

 red between crossed nicols, is placed with its 

 axis parallel to the lines of strain of the 

 celluloid strip, the succession of colors is red, 

 orange, yellow, green, blue or retrograde. The 

 quartz effect is thus gradually more and more 

 neutralized by the celluloid strip as its strain 

 increases. If the axis of the quartz plate is 

 at right angles to the lines of strain, the order 

 is red, purple, blue, green, yellow or direct. 

 The quartz and celluloid effect coincide in 

 sign. Hence the ray vibrating in the direc- 

 tion of the lines of stress corresponds to the 

 ordinary ray in quartz, which from the positive 

 character of the wave surface, is the swifter. 

 It follows that the ray vibrating parallel to 

 the lines of stress of the celluloid strip moves 

 with greater velocity than the ray vibrating 

 normally to this direction. In other words 

 the extraordinary ray is swifter and the wave 

 surface of strained celluloid is negative: for 

 on inclining the celluloid strip around the 

 lines of stress as an axis, the succession of 

 colors (due to increasing thickness) is direct in 

 order; whereas on inclining the strip around 

 an axis normal to the lines of stress the suc- 

 cession is actually retrograde, showing that 

 the optic axis is being rapidly approached, 

 where double refraction ceases in spite of 

 thickness. 



Similarly if a quarter wave plate of mica 

 is inserted with the effective axis parallel to 

 the lines of stress, the order of colors for 

 crossed nicols is bluish, dark (neutral, com- 

 pensation), bluish, yellowish, etc. (excessive 

 celluloid effect), or clearly retrograde. If in- 

 serted with the axis normal to the lines of 

 stress the succession of color is bluish, yellow- 

 ish, red, purple, blue, etc., or direct, aU of 



which admits of the same interpretation as in 

 the case of quartz. 



An equally interesting compensation is ob- 

 tained by crossing two similarly stretched 

 strips of celluloid at right angles. In this 

 case the area where the strips overlap is quite 

 neutral (dark between crossed nicols) while 

 the four non-duplicated areas, extending out- 

 ward from the square center, are vividly 

 colored. The faster ray in one strip becomes 

 the slower in the other, and vice versa. 



My thanks are due to Professor Barus, who 

 suggested these experiments, for his aid 

 throughout the whole course of the work. 



Lulu B. Joslin. 



Bbown University. 



AMCEBA BLATT^ AND AMCEBOID MOTION. 



The writer wishes to call the attention of 

 teachers of biology to a form of Amosha that 

 hitherto has been somewhat neglected in this 

 country, but which is of much theoretical in- 

 terest. It is, moreover, sufficiently plentiful 

 and easily obtained at all seasons of the year 

 to be adapted to the uses of small laboratory 

 classes. This is Amoeba llattw Biitschli, which 

 inhabits the intestine of the croton bug, or 

 common cockroach, Blatta (Phyllodromia) 

 germanica, a well-known immigrant from 

 Europe that has established itself in our 

 larger towns and, at least in the eastern states, 

 in many country villages. Throughout the 

 year this cockroach is active in bake-shops, 

 creameries, sugar refineries and in the kitchens 

 and basements of hotels, restaurants and pri- 

 vate houses, where it may be found under 

 sinks, about water pipes and in similar warm, 

 dark places. 



Ehumbler,' in a recent paper in the ' Fest- 

 schrift ' commemorating Professor Ernst 

 Ehler's seventieth anniversary, describes fuUy 

 the movements of this AmM:ha, as well as the 

 methods that have been employed at Gottingen 

 for obtaining it. The cockroaches are ether- 

 ized, the heads and terminal segments of the 

 abdomen clipped off, and the intestine care- 



'■ Rhumbler, L., '05, ' Zur Theorie der Ober- 

 flachenkrafte der Amoben,' Z. f. wiss. Zool., 83 

 Bd., pp. 1-52. 



