252 THE AMERICAN MONTHLY [Novembei-, 



observation. It is sometimes desirable to use a long standard distance, 

 e. g.^ in drawing objects under the microscope for illustrating a paper 

 or a lecture. Another instance in which you may with advantage 

 place your ocular about two feet above the table is in measuring mi- 

 nute objects, such as blood corpuscles, or the cells of the yeast-plant, 

 with a 1-6" objective, or one of Zeiss's D's. Here, by projecting a large 

 image, you can obtain more reliable average measurements than if the 

 standard of lo" were adhered to. You may, on the other hand, wish to 

 have your sketch on a smaller scale than is to be secured by drawings 

 at the standard distance of lo inches. Small sketches are sometimes 

 desirable in publishing illustrations. All you have to do is to bring 

 your eye-piece nearer to your paper until you attain the required 

 dimensions. You can in this way applv the law I referred to, and 

 make your sketches exactly double or treble, or exactly half or a quar- 

 ter of the size secured at the standard distance. An engraver can, of 

 course, enlarge or reduce a drawing, liut by adopting the methods I 

 describe, you avoid all errors in proportion of parts which might be in- 

 troduced in an enlarged or reduced sketch, for you draw dii'ect from 

 the object itself. The inconvenience of being always able to secure 

 the precise distance of lo" is easily disposed of by getting a stand, 

 which will cost you only a few annas, and last forever. Cut a cube 

 of teak, or other heavy wood, to a heiglit which, added to the distance of 

 the centre of your eye-glass from the table when the tube is placed in 

 the horizontal position used in drawing, will give a total of ten inches 

 from eye-piece to paper. It would be a move in the right direction if 

 microscope makers put their instruments up in cases which could be 

 utilized as stands for securing the lo-inch distance in drawing and 

 micrometry. You will find the block I have described come in handily 

 for many purposes when you have it made ; one that I have, ot a 

 special pattern, does duty for a drawing stand, a dissecting microscope, 

 and an apparatus for hardening balsam in balsam mounts Moreover, 

 by always having such a block beside you, you will make more 

 measurements and draw more of your objects than you would if you 

 had to find the standard height for each sitting. 



You will find it convenient to make a table giving the magmfymg 

 power of each of your objectives with each ocular, and tor different 

 fixed tube-lengths. You are thus enabled, without constantly referring 

 to the micrometer, to specify (as you always should) the magnitying 

 power employed in your different sketches ; it should be noted beside 

 the sketch thus, X 150, which means that the sketch has been drawn 

 with a power of 1^0 diameters. In this connection I may tell you that 

 in noting or speaking of the magnifying power of a microscope you 

 should always refer to the number of diameters it magnifies. More 

 meaning is conveyed when you say a microscope magnifies 100 diame- 

 ters than when you vaguely say it magnifies 10,000 times. You can- 

 not expect people to work out sums in square root whenever you tell 

 them your microscope magnifies so many times. 



Again, there are tw^o terms in microscopv which should be carefully 

 defined in our own minds, and which should be carefully emploved : I 

 refer to the terms mag7iificatio7i^ or amplification, and maguifying 

 power. By magtiiji cation^ or amplification, is meant the relation be- 

 tween the real size of an object and of its image when projected by any 



