BRITISH LICHENS. 85 



Microscopic 3Ica.iuring of spores is such an important clue to the 

 identification of species that it cannot be altogether passed over here, 

 although, for fuller details, we must refer the student to works treating 

 specially on the use of the microscope. Several methods are in use, 

 and each worker will prefer his own ; but, if the student is starting 

 de noi-o, we would recommend the method we ourselves use, which 

 is as follows : — Procure a stage micrometer, ruled for one-thousandths 

 of an inch, or of a millimetre, whichever is intended to be adopted, 

 and, placing it on the stage of the microscope, under the object glass, 

 place at the same time a sheet of white paper at an equal distance 

 from the eye, which will be about nine or ten inches, on a board 

 adjusted for the purpose. Having done this, look down the tube at 

 the micrometer scale with one eye and at the sheet of paper with the 

 other simultaneously, when the image of the micrometer will be 

 thrown on the paper, and lines may be made on it corresponding with 

 those of the micrometer. The first will be a rough copy, and should 

 be revised, by accurately ruling the divisions on the margin of a piece 

 of white cardboax'd. When this scale has been successfully made, the 

 stage micrometer can be dispensed with as no longer necessary. The 

 scale will be kept close at hand, and will be used as follows : — 

 Supposing the object to be measured be a spore under observation, a 

 diagi'am should be made in precisely the same manner as the scale 

 was made, namely, by looking with the disengaged eye on a blank 

 piece of paper, and sketching the image thrown on it. When the 

 sketch is completed, and ascertained by repeated trials to correspond 

 with the apparent size and shape of the spore, the scale can be applied 

 to it with ease, and the dimensions ascertained. A separate scale must 

 be made for each power used. 



The scale usually adopted in this country is the divisions of an inch, 

 while the rest of Europe adopts the divisions of a millimetre. We 

 prefer the latter, as most Lichenologists use it, besides which fewer 

 figures are required to denote the quantities ; thus, if we require to write 

 the dimensions of the spore of Placodium mururum (Hffm.) in millimetre, 

 we do it thus :— -Oil - -014 x -007 - -008 mm. ; while, if written in 

 divisionsof an inch, it is %vrittenthus :— -00033 x 00045 x 00027 - OOOSlin. 



Supposing the student to have mastered any difficulties he may 

 have thus far met with, he will have acquired so complete a knowledge 

 of the plant he may have under examination at this stage, that he will 

 be in a position to assign it to its place, and append to it its proper 

 generic and specific name. That he will meet with difficulties there 

 can be little doubt, but we advise patience, diligence, and care, before 

 which the most formidable must disappear, and without which nothing 

 worth doing in this world can be accomplished. 



