_Figs._ 1. and 2. Transverse section of the human clavicle (collar bone), showing Haversian canals, concentric laminæ, and concentric arrangement of bone cells--3. Transverse section of the femur of an ostrich--4. Transverse section of humerus (fore-arm) bone of a turtle, Chelonia mydas--5. Horizontal section of the lower jaw-bone of a conger eel, in which no Haversian canals are present--6. A portion of the cranium of a siren, Siren lacertina--7. Portion of bone taken from the shaft of humerus of a Pterodactyle, showing elongated bone-cells characteristic of the order Reptilia--8. Horizontal section of a scale, or flattened spine, from the skin of a Trygon (sting-ray), showing large Haversian canals, numerous wavy parallel tubes, also bone-cells with canaliculi communicating as in dentine. ERRATA. Prefaces, page vi., line 22 _Insert_ “a” _into_ “admitted.” " " x., line 13 _Insert_ “the” _before_ “assistance.” " " xii. _Insert_ “for” at commencement of line 24. Page 33, line 13 _For_ “Rabbit” _read_ “Kitten.” " 486, 4th line from bottom _Strike out_ “The late.” " 511, two lines from bottom _For_ “Plate XIII.” _read_ “Plate XVI.” " 584, 5th line from bottom _Insert_ “Stalk-Eyed” _before_ “crustaceans.” " 624, line 12 _For_ “or” _read_ “and.” " 633, Plate VII. Numbering of figures-- _For_ “152” _read_ “158.” _For_ “152a” _read_ “152.” (Professor Abbe, erroneously referred to more than once as “the late” is, the author is happy to say, in excellent health). THE MICROSCOPE. PART I. Early History of the Microscope.