GENERAL CHARACTERS OF BRITISH LICHENS. 



59 



as to require frequently a careful examination for their de- 

 tection : they usually coexist on the same thallus with, and in 

 proximity to, the apothecia, and occur in the form of tuber- 

 cles or dark points, which the older Lichenologists described 

 as epithalline growths, parasitic Fungi, etc. The latter, the 

 Pycnides, are similar organs, occurring however only in a 

 very few minute parasitic Lichens. To acquaint ourselves fully 

 with the structure and constituent elements of these various 

 organs, it is necessary to apply comparatively high powers 

 of the microscope."* 



* Thanks to competition in the manufacture of optical instruments in 

 Britain and on the Continent, the student can now provide himself with an ex- 

 cellent compound microscope, at a comparatively low price. Among the best 

 and cheapest instruments are those made by Nachet and Oberhauser, of Paris, 

 which can be procured for from £5 upwards. Those reaching a magnifying 

 power of four hundred to six hundred diameters, should be selected by the 

 student whose object is the investigation of the minute anatomy of the 

 Lichens ; for ordinary studies in vegetable anatomy, instruments reaching a 

 power of two or three hundred diameters are sufficient. Most British opticians? 

 by using French or German lenses, can now supply excellent instruments 

 at a moderate price ; and in consequence of prizes recently offered by the 

 London Society of Arts, for the best and cheapest students 3 microscopes, a 

 Birmingham firm (Messrs. Field) advertises them at a price ranging from 

 10*. 6d. up to £3. Among the most celebrated London makers are Ross, 

 Powell and Lealand, and Smith and Beck ; and in Edinburgh, Messrs. Bry- 

 son have had much experience in their construction. 



