LESSON 1.] INTRODUCTION. 3 



stituting the outer wall, or integument, as transparent as glass, its 

 surface covered with minute green dots (5), with several spheres of 

 variable size, and much darker green contained within it (a), con- 

 tribute to form an object of unequalled F 

 beauty ! 



14. Neither is this all ; those per- 

 sons who see it for the first time, will 

 be surprised to find that it (a plant) is 

 locomotive. 



15. It will constantly be seen roll- 

 ing to and fro, with a remarkably 

 steady, equable motion; no obstacle 

 retards it nothing accelerates its 

 speed : frequently it occurs that two of 



them are seen approaching each other 



Volvox globator. 



in opposite directions ; collision appears 



to be imminent, but just at the instant of anticipated conflict, one 

 turns off to the right, and the other to the left, and each pursues 

 its way. 



16. The locomotive organs are not easily seen, even by the Mi- 

 croscope two essentials are necessary ; firstly, an excellent fourth 

 achromatic object glass,* and, secondly, careful and judicious man- 

 agement of the light. These conditions being established, a number 



* Certain expressions have been used in this work which require explanation; thus, in 

 speaking of the magnifying power employed, "a fourth," or "an eighth " object glass is men- 

 tioned. 



An Achromatic object glass, if of French construction, consists of three plano-convex 

 lenses; if English, of three pairs (six) of lenses. The focal length of such combinations, 

 always short, may yet be of variable length, while the magnifying power may be the same, 

 therefore focal length forms no indication of magnifying power. 



For the perfect demonstration of the latter, it is usual to use the achromatic combination 

 as a single lens, and test its exact capabilities by comparison with a single plano-convex 

 lens, and then to name the former by the focal length of the single lens, with which it 

 agrees. 



An "inch " achromatic means that the glass is equivalent to a plano-convex lens with an 

 inch focus in other words, that it magnifies exactly ten diameters: a "fourth," or an 

 " eighth," mean that they are equal to plano-convex lenses, of one-fourth or one-eighth of an 

 inch focus, or 40 or 80 diameters. 



In the Compound Microscope, there are two additional modes of obtaining an enlarged 

 image, beyond the use of the object glass ; these are, the length of the tube of the compound 

 body, and the eye-piece. The legitimate way to obtain power is by the object glass, as, 

 although the other two plans may greatly enlarge the image, they are powerless to show 

 structures that the object glass fails to develop. 



The great number of diameters pompously claime ^ oy some authors, depends entirely 

 upon the length of tube and depth of eye-piece both of them objectionable (frequently falla- 

 cious) modes of obtaining magnifying power. The best and fairest way is to state the value 

 of the object glass employed. 



