56 



MICROSCOPIC OBJECTS. 



exhibit the spiral nature of its structure. Mr. Pritchard, 

 from whose preparations these engravings were made, 

 remarks in the ' Microscopic Illustrations ' that this 



Fig. 5. Fig. 6. Fig. 7. 



structure admh-ably illustrates the interference of light. 

 For this purpose the shde containing the tracheae should 

 be held close before the eye, when looking through it 

 at a lighted candle about a yard distant ; the coloured 

 images will then be seen on each side of the real one. 

 From the distance at which these images are apart may 

 be deduced the diameter and the distance of the spiral 

 fibres from eax^h other. 



In the perfect beetle of this larva, the spiraculae are 

 arranged along each side of the back under the wings, 



