OF MICROSCOPIC OBJECTS. 21 



any amount of soaking to render them sufficiently trans- 

 parent, some bleaching process should be premised. A for- 

 mula for such a process , may be found in another part of 

 tli is work, where the preparation of the antennae of insects 

 is described. If that should not prove successful, some 

 modification will easily occur to the student. Of course it 

 is not all insects that can be treated in this way, the size 

 and deep colour of very many quite preventing a good 

 result; but when they have been successfully prepared by 

 any of the methods of which we have spoken, it is then 

 possible to discriminate their internal organs by the differ- 

 ences of colour which they present. The use of the binocular 

 microscope, and of objectives of low angular aperture, will 

 also much facilitate this mode of examination, by increasing 

 the depth of focus, and enabling the organs to be seen more 

 or less in connection with each other, even if they be super- 

 posed. It is also possible to examine the muscles of the 

 limbs and bodies of insects, so as to decide upon their forma- 

 tion, origin, and insertion, and probable mode of action ; and 

 this is only one of many such uses. What a mistake must 

 it be, then, to prepare insects for mounting by boiling in 

 liquor potassse, and so dissolving out their viscera, and 

 squeezing them flat ! 



In the case of living insects, especially those of the more 

 transparent salt and fresh water species, the results of their 

 examination by polarized light are exquisitely beautiful and 

 interesting, because their organs and circulation may be 

 more clearly discriminated while in motion. 



10iH DIVISION. 



Electricity has been employed in histology partly for its 

 electrolytic effects, but chiefly as a means of producing 

 certain variations of temperature in objects under examina- 

 tion. Strieker says " that the tissues become altered by it 

 as they would be were they subjected to the action of weak 

 acids or alkalies," and he describes a rather complicated 

 apparatus for this purpose, of which it is impossible to give 



