120 SUMMARY OF CURRENT RESEARCHES RELATING TO 



diffraction theory of microscopical vision to the examination of such 

 minute ohjects as muscle-fibre. 



In tho first place, the examination by the instrument of musclo from 

 the femur of Hydrophilus piceiis showed, beyond a doubt, that the con- 

 tracted portions of a fibre have a higher refractive index than the 

 remainder ; but, on the other hand, Prof. Exner claims to have proved 

 that this is only the caso with abnormal contraction, whereas when the 

 contraction is normal, no change is produced in the refractive power. 

 The immersion fluid used to determine the index was either whito of egg 

 concentrated over sulphuric acid in the receiver of an air-pump, and 

 treated with acetic acid, or the liquid obtained by pressure from the eye 

 of an ox or sheep. The refractive index of the former can be raised to 

 1-4053, and that of the latter to 1- 42-1 -43. A number of trials with 

 these fluids led to the result that the stationary living muscle of Hydro- 

 2>hilus has an index of refraction which varies slightly on either side of 

 the value 1*363, while the same muscle may have slightly different 

 values in different parts. As regards what may be called the ordinary 

 and extraordinary rays for light traversing the fibres in a direction 

 perpendicular to their length, measurements of the indices in the 

 sartorius muscle of a frog led to the approximate values n = 1 • 368 for 

 the ordinary ray, and n e = 1 • 370 for the extraordinary ray. 



When the screen of the micro-refractometer is placed with its edge 

 at right angles to the length of the fibres, a peculiar striped appearance 

 is produced, which the author explains as due to the obliquity of the 

 lajers constituting the fibre, so that a ray of light is deflected or not 

 according as it does or does not pass through layers of varying refractive 

 index. Now when the waves of contraction which traverse the living 

 muscle of an insect isolated in an inactive fluid of equal or greater 

 refractive index are examined with the micro-refractometer, the screen 

 having its edge parallel to the length of the fibres, it is found that the 

 contracted portions become dark on the side of the screen and light on 

 the opposite side, in other words, the index of refraction in these parts 

 is diminished ; if the index were increased, the first effect would be an 

 illumination of the fibre as far as the sarcolemma, and this is never 

 observed. 



On the other hand, the permanently contracted and transversally 

 striated parts found in fibres which are still living, especially near the 

 torn ends, do exhibit a marked increase of refractive power; these, 

 however, are regarded by the author not as normal contractions but as a 

 change which accompanies the death of such parts of the fibre ; they do 

 not recover their previous character, because the muscular substance has 

 been partially destroyed, and this is proved by three facts — (1) the 

 permanently contracted parts are smaller than those of which the con- 

 traction is normal. (2) the death of a fibre is accompanied by the 

 emission of a certain amount of liquid, as may be proved by examining 

 the fibre in liquid paraffin (refractive index = 1*4712), when the micro- 

 refractometer indicates that the contracted portion is surrounded by a 

 liquid of less refractive index than the paraffin ; (3) it is only necessary 

 to examine a free fibre under the Microscope, when it will be found 

 after a few hours to have contracted and to be surrounded by liquid, 

 and a contracting portion may be occasionally seen during a few minutes 

 to surround itself with a ring of liquid as it contracts. 



It may be concluded therefore that there is an absolute distinction to 



