34 BRITISH LICHENS. 



scopist, though there is no objection to such slides, but we mean that it 

 should clearly display the essential parts of the object under examina- 

 tion, a quality that may co-exist with a certain rudeness of finish in 

 varnish and ornament. We will here lay befoi'e the student a few 

 practical directions for pi-eparing his slides, as in so doing we shall be 

 able to convey incidentally some information on other practical points 

 in examining specimens. 



Cuttinfj Section-t is a delicate and difficult operation, on which much 

 depends in seeing clearly the structure. The excellence of a section 

 consists in two things — first, being sufficiently thin to allow of transmitted 

 light passing easily through all parts, so as not to require so much 

 pressure as would derange the relative position of the various parts ; 

 and second, it should embrace the whole, or as much as possible, of what 

 is required to be seen, so as to render unnecessary a second section being 

 made. If the object were to mount a large number of slides, a good 

 section cutter should be obtained from some respectable optician. All, 

 however, that is required where a single section is to be made can be 

 accomplished with a small dissecting knife, having a thin, penknife edge, 

 with a fine point. A watchmaker's eye-glass, having a lens of lin. 

 focus, held to the eye by contracting the muscles of the face, or, when 

 this cannot be done, supported by an elastic band passing round the 

 head, will be found convenient in making a section, as the two hands 

 are left at liberty for work. Should it be the apothecium of a Lichen 

 that IS to be examined, the Lichen is held in the left hand, and a clean 

 perpendicular cut is made near the middle of the apothecium, and the 

 portion nearest the eye is removed, so as not to obstruct the sight, when 

 a second cut is made parallel with the first, by which the thinnest 

 possible section is obtained. Success will not reward the student till 

 several apothecia are destroyed. He must practice, therefore, on some 

 common species. When a sufficiently thin section is obtained, let it be 

 placed on a glass slide, if for present examination only, in a drop of 

 hydrate of potash, which loosens the parts to be examined, making them 

 more transparent ; but if for mounting permanently, in a drop of clean 

 water, as the " medium " used for mounting would be injuriously 

 affected by hydrate of potash. Placing the covering glass over the 

 section, and subjecting it to moderate pressure, it is now ready for 

 examination. The magnifying power best adapted for Lichens is, as 

 already stated, one of about 350 to iW times linear. 



A Medium for Moiuitinf) Speciiiu-ii.-i should be at hand in case the 

 section is exceptionally good, or the specimen too valuable to admit 

 of more than one section being taken from it, in which case it can 

 be at once permanently mounted. A very useful medium is that 

 prepared by P. Aylward, of Manchester, which requires to be warmed 

 to render it liquid ; and on cooling is sufficiently firm to admit of the 

 surplus being washed off from around the covering glass, which is held 

 in its place by a light clamp. One serious defect of this medium is 

 that it does not preserve the colour of the chlorophyll in the gonidia. 

 The slide should now be finished off, as directed in the ordinary works 

 (;n microscopic mounting. 



