MICROSCOPIC OBJECTS. 29 



nature of these air-tubes, together with the numerous 

 openings in the membranes, are observable. A drawing 

 of this larva (magnified) is published in the "Notes on 

 Natural History," by Mr. Pritchard (Plate 3.) Of late 

 years the species of Libellula possessing these leaflet-tails 

 has become scarce about London. 



Anatomical Preparations. — In the Hst, only a few inte- 

 resting subjects are named : others wiU readily suggest 

 themselves to persons engaged in such pursuits. 



The medical student will find the achromatic microscope 

 an important instrument in these researches ; and since 

 its cost has been so greatly reduced (see the concluding 

 chapters) the difficulty of procuring its aid has been 

 materially lessened. For the medical practitioner who 

 desires to investigate the states of disease or the nature 

 of urinary deposits, the vertical achromatic microscope 

 is amply sufficient ; and it will be found wise not to en- 

 cumber himself with an unnecessary number of magni- 

 fiers. An object-glass of one third of an inch focus will 

 in most cases be sufficient. 



The subjects named in the list have their vessels 

 injected with different-coloured substances. They are 

 mounted as opaque objects under thin glasses about half 

 an inch in diameter. To view them efficiently, the light 

 should be condensed upon them, and the power used 

 should be from 35 to 80 times, linear. 



Antenna. — The horns of insects not only vary very 

 much in form in different genera, but those of the male 

 and female differ in the same species. In the Catalogue 



