122 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



c 



fluid similar to that surrounding the fibre, an effect is produced more 

 or less similar to that of the air-charged fibre, for if the refractive power 

 of the contained and the surrounding fluid is greater than that of the 



solid walls, tho latter will appear 

 Fig. 27. as hollow spaces in the stronger 



refracting medium, as would be 

 the case with glass capillary 

 tubes filled and surrounded with 

 monobromide of naphthalin. 



If oblique illumination is 

 employed instead of central, the 

 appearances just described are 

 not essentially altered ; a dis- 

 placement of the illuminated 

 line to the one side or the other 

 is simply produced, according as 

 the mirror is moved out of the 

 axis to the right or left. With 

 objects which act as convex 

 lenses it is generally displaced 

 to the side of the object which 



Glass capillary tubes. Focus, A. medium, 

 B lower, C higher. 



is turned away from the source of light, and with objects acting as 

 concave lenses to the side nearest to the light ; and therefore, as the 

 compound Microscope inverts, it will appear in the first case on that 

 side of the image which is turned towards the mirror, and in the latter 

 case away from it. The glass thread or the solid fibre will therefore 

 show the line of light on the side turned towards the mirror, when the 

 illumination falls obliquely and the tube is raised ; hollow cylinders 

 and furrows will show it, when the tube is lowered, on the side of tho 

 image which is turned away from the mirror. The division of light and 



Glass threads with oblique light 

 incident from the right. Focus, 

 A high, B somewhat lower. 



Glass capillary tubes with oblique 

 light incident from the right. Focus, 

 A low, B a little lower. 



shadow will appear as in A, figs. 28 and 29. If a more medium focus 

 is taken, the conditions are so far altered, that now half of the object is 



