xxviii INTRODUCTION. 



Divided Scale. A metallic or ivory scale divided into lOOths &c. of an inch, is indis- 

 pensable in micrometric admeasurements (see MEASUREMENT). The metal or ivory 

 should extend beyond the graduated portion. 



Micrometer. A glass slide with fine lines scratched upon it with a diamond, these being 

 loWtn of an inch apart, is absolutely requisite. Another, with coarser divisions, is also 

 required to be placed in the eyepiece, for making measurements (see MEASUREMENT). 



A rectangular brass table, with two legs at one end and one at the other, is useful in 

 macerating objects upon slides iii chemical reagents, oil of turpentine, or Canada balsam, 

 and in mounting objects. It is heated by a small spirit-lamp placed underneath. 



Ring-Net. A very useful piece of apparatus for collecting Desmidiaceae, Diatomacete, 

 &c., where entangled amongst Confervre &c., or forming crusts or films upon other aquatic 

 plants, consists of a brass or wooden ring about 4 inches in diameter, furnished with a 

 groove round its circumference, in which also a radial aperture exists, through which 

 the end of a stick may pass. A piece of very fine muslin, rather larger than the ring, is 

 then laid over it, and the margins of the muslin fixed in the groove by means, of a vulca- 

 nized Indian-rubber band. Or this apparatus may be so modified, that the muslin is fixed 

 by means of an inner ring, adapted to the outer, but incomplete at one point of its circum- 

 ference, and with a projecting rim to prevent its passing through the outer ring. Thus \\\\ 

 have a kind of strainer; and by using several pieces of previously wetted muslin in >iu - 

 cession, a large number of the minute organisms may be separated from the water. The 

 pieces of musliu may be brought home, folded up, in wide-mouthed bottles, separately. 01 

 several in one, according as the organisms are obtained from one or several waters. In 

 this way we save carrying a large quantity of water. The pieces of muslin are afterwards 

 opened and placed in jars of filtered river-water, and exposed to the light, when the 

 organisms will become detached. 



A simple microscope, or some apparatus which will allow of dissection with the aid of 

 lenses, is essential, although the erecting eyepiece or the erecting-glass (p. xxii) will answer 

 the same purpose. It is of little consequence which be selected, provided a large and firm 

 sloping arm-rest be furnished on each side of the stage. Either doublets or the lower 

 powers may be used. Some of the modem simple microscopes are binocular, 



Leather Case and Collecting- Bottles. The Diatomaceee, Desmidiaceae, and other smaller 

 Algae, as also the Infusoria, require to be collected and brought home in bottles. These 

 should be of about 1 or 2 ounces capacity j and, for portability without risk of being 

 broken, they should be packed in a case made of stout leather, with a separate space for 

 each bottle. The whole will pack up in the form of a book. 



Having given a sketch of the most important pieces of apparatus, we will say a few 

 w-ords upon the illumination. 



Ittummation. The best light in general for microscopic purposes is undoubtedly day- 

 light, or that of the sun reflected from the clouds j and this is certainly the light which 

 can be borne for the greatest length of time without injury to the sight. The position of 

 the observer is of importance ; it should be such that the window is on his left hand, or 

 even the back slightly turned towards the window. The advantages of this position are 

 great ; for then but little light will enter the eyes directly from the window, and it is of 

 the greatest importance, during a microscopic examination, that the least possible amount 

 of light should be admitted to the eye, from any source, besides that transmitted through 

 or reflected from the object. In drawing also with the camera lucidathis position should 



