184: THE AMEEICAN MONTHLY [October, 



green has been caught on the sHde scrape it into a heap in the centre, place 

 a ch'op of water on it, then gently cover it with a circle. Examination with 

 low power (50 diameters) will not show much beyond a heap of minute, 

 greenish grains, and the high power (300 diameters) must be used at once. 

 It will show that the fine powder is made up of fine grains, some of which 

 are collected into masses and several others independent. It is better before 

 attempting to unravel the facts from the tangle of plants before one to sepa- 

 rate as well as possible the mass. This might have been done by teasing be- 

 fore the cover had been put on, but can be better done by tapping the cover- 

 glass. To do this take the teasing needle and, holding it vertically over the 

 cover, hit it firmly and forcibly several times just above the heap. This will 

 soon spread out, and a continuation of the tapping will effect far more than 

 teasing could in this instance. To make the tapping successful mount the 

 specimen carefully thus : — drop on, before covering, less water than enough 

 to fill out the space beneath the cover, and then add from the dropping tube 

 enough to just fill out this space. The tapping will, perhaps, destroy a few 

 of the plants, but it will for the most part only spread out those which would 

 otherwise produce indistinguishable confusion. In case the first slide pre- 

 pared is not successful, do not hesitate to try it again and again, thoughtfully 

 profiting by failures. There is no royal road to learning this ; it is a matter 

 of hand-training, and with repetition is bound to come. 



The slide, when prepared properly, will show a great variety of little 

 globules compounded in many w^ays, besides several other matters not be- 

 longing to the plant. It is easy to rule out as not belonging to the study all 

 bodies not bovmded by spherical walls and containing green matter. All the 

 spherical bodies with green are Protococcus. You must at once try to neg- 

 lect everything else in the field and concentrate your attention on the plant. 

 To do this is not so easy at first, and it w'ill seem, perhaps, as interesting and 

 curious to examine numerous other things which may be in sight. A glance 

 at Protococcus will show that it is made up of bodies which are bounded by 

 curved lines of black or white, according to the manner of focusing. A great 

 variety is now. found — here single circles, there several, three or four or more 

 Loosely adhering ; again, ovals and ovals with indented sides and a line across 

 them. Selecting one of the circles, examine it first. The sharp line which 

 bounds the mass within can be seen. Perhaps just within this line there is 

 a light space. These two appearances may be reversed by focusing, and 

 are due partly to behavior of the light as its course is modified by the 

 boundary line of the plant. Within this boundary line may be seen a green 

 substance pervading the entire body. This, on more careful study, is found 

 to be not homogeneous even in texture, but is broken up into lighter and 

 darker spots. This appearance is designated granular. In the centre of the 

 granular substance there can usually be seen a green body of homogeneous 

 appeai^ance. Careful study is required to bring this central body into view. 

 The parts thus seen are the outer boundary, called the cell-wall (Fig. i,c.w.), 

 the green substance, called the cell-contents (Fig. i, c. c), and the central 

 nucleus (Fig. i, n.) These are the necessary parts of a Protococcus plant, 

 and this single body is a complete plant, capable of anything which it is pos- 

 sible for Protococcus to perform. It is necessary to call the especial atten- 

 tion of beginners to one question at this point, viz., the shape of the little 

 body he is studying. It must be remembered that the microscope deals only 

 with surfaces, and that it may require a nvunber of views to tell the truth as to 

 the real shape of a body. Protococcus, so far as revealed by the appearance 

 of figure I, inight be either a disk or the central plane of a sphere. To the 

 microscopic vision things out of the focal plane are utterly invisible, and 

 an opaque body not large enough to entirely cover the object-glass can be 



