1888.] MICROSCOPICAL JOURNAL. 221 



oblong thin metal plates luveted together, clamping the magnifying lens be- 

 tween them near the upper margin. As the plates were sometimes made of 

 silver, we may suppose that it was choice, not necessity, that induced him to 

 make his own optical tools ; as a rule, however, they seem to have been of 

 brass. On both sides of the glass lens the plates were pierced by a round 

 hole not larger than the head of a small pin. The eye, when about to ex- 

 amine an object, was placed close to the microscope, so that it could look 

 through these holes and through the lens at the specimen on the opposite 

 side as the instrument was held in the hand and towards the light. On the 

 back, as shown in fig. 3. there is an upright, pointed rod just opposite the 

 lens. On this the object to be studied was fastened, usually by being pressed 

 against a little piece of wax, and the focus was obtained by raising or low- 

 ering the rod by the long sci"ew shown in fig. 2 ; the smaller one near the 

 centre of the metallic plate being used to remove the object from the lens, or 

 to bring it nearer. When the parts had been properly arranged, Leeuwen- 

 hoek studied the specimen by gazing at it long and earnestly through the 

 little lens which his own hands had made. His success is due to his inex- 

 haustible patience, his tireless study, and. to his skill in understanding and 

 correctly describing the thing he saw ; with such magnifying glasses he must 

 have seen the objects very impeiiecth' and indistinctly. 



In the following paragraphs are enumerated a few things which no human 

 eye had ever seen until the eye of this old Dutch microscopist saw them. 

 Some of his discoveries are so well known, and have so generally become 

 the common property of students, and they refer to so common objects, that 

 it seems as if even beginners in microscopy ought never need to learn them. 

 Those wh* have used the microscope much can scarcely remember when 

 they learned about some of these things, and the modern student often feels 

 surprised when he hears that what he may see almost anywhere, and at al- 

 most any time, were unknown to the more advanced observers of natural 

 science in the seventeenth century. 



Among his discoveries which readers are least likely to know about is one 

 in reference to the eggs of the common muscle ( Unio) which every boy has 

 found in the mud of shallow streams. Leeuwenhoek was the first to learn 

 that it lays eggs, the first to see the young muscles within the egg, and the 

 first to discover that these little unhatched animals do not lie quietly inside 

 the delicate egg, but that they are continually turning round and round. He 

 was himself so surprised by this continuous motion, that he wrote : — ' This 

 uncommonly pleasing spectacle was enjojed by myself, my daughter, and 

 the engraver, for three whole hours, and we thought it one of the most de- 

 lightful things that could be exhibited.' To this day it is a delightful thing 

 to see under the microscope, and the owner of an instrument, with even a 

 low-power lens, can see something very like it at almost any time during the 

 spring or summer, and in the winter, too, if he keep an aquarium with a 

 few water-snails in it. This rotation of the young inside the egg is not con- 

 fined to the eggs of the muscle. Those of any water-snail will show it beau- 

 tifully. Snails' eggs are plentiful in any pond. They are to be found attached 

 to floating chips or submerged logs. To the naked eye they resemble an ob- 

 long or rounded mass of colorless jelly, perhaps an inch, or less, in length, 

 with many little dark spots scattered through it. These spots are the very- 

 young snails within the delicate eggs which the jelly surrounds and protects. 

 In fig. 3 is shown a jelly mass of this kind, about the natural size, the scat- 

 tered black dots representing the young snails as they appear to the naked 

 eye. The jelly is quite firm, and can be scraped oft" with a knife-blade. It 

 may be placed in a deep cell for microscopical examination. Then each egg 

 will appear with the young snail within it, somewhat as is shown by fig. 4. 



