223 



(2) A more successful way to exhibit the presence of the 

 double spiral is to make a camera lucida drawing of the upper 

 ■striations of a muscle fibre; then foeussing through to the 

 lower side make a similar drawing on a separate piece of fairly- 

 transparent paper. Accurate superposition of the two draw- 

 ings will reveal the presence of a single spiral. Care is 

 required in the interpretation of the result. Only one-quarter 

 of a turn of a spiral is visible in one focus of the micro- 

 scope ; by focussing successively, therefore, upon the upper 

 turns and then upon the lower turns, the additional rise or 

 -drop in the spiral which would occur if the path of the spiral 

 in the thickness of the fibre could be observed, is eliminated. 

 The single spiral obtained by the superposition of the two 

 <lrawings will therefoTe indicate the actual presence of a 

 ■double spiral. 



The objection which may be taken against this inter- 

 pretation is that the change in direction of the striations in 

 the upper and lower side of the muscle under examination is 

 due to a shearing stress, perhaps due to the pressure of the 

 cover-glass on the preparation. By no conceivable method of 

 •distortion, however, can transverse striations be converted 

 into real spirals, and the fact that it is possible to travel 

 along the spiral in stretched fibres by successively focussing 

 up and down along it, eliminates this difficulty. 



Moreover, it is possible, in focussing through a fibre 

 •suddenly to come to a focus where there is a discontinuity — 

 very faint, but still perceptible — between the upper and lower 

 portions of the ''transverse" striation, and it is often possible 

 (see figs. 1 and 2) to observe at one focus the crossing of 

 ■striations at this point, clearly indicating their spiral nature. 

 Neither is definitely in focus, and while it is possible to see 

 both at once, neither can be observed sharply. This is to be 

 noticed especially clearly at the terminations of fibres, or in 

 those places where they are not quite flat, but where the fibre, 

 first in focus, bends slightly out. Under these circumstances 

 one obtains a partial view along the longitudinal axis of the 

 fibre, and can at one focus obtain a view of a partial turn of 

 the spiral. 



It is, perhaps, necessary to add that care must be taken 

 not to focus along the plane of contact of two superimposed 

 fibres; without this precaution a false crossing effect might 

 readily be obtained by the simultaneous indistinct focussing 

 of the striations of two fibres. 



For a clear demonstration of the double spiral the longi- 

 tudinal body muscles of chalcid wasp larvae may be 

 recommended . 



