39 



In its construction the objective is immersed in water, that 

 is, there is a film of water between the front of the objective 

 and the object, or thin glass covering it. An amplifier, is an 

 achromatic combination inserted in the compound body of the 

 instrument, to increase the power of the objective and eye- 

 piece. Angle of aperture is the angle in the surface of the 

 front lens, at which light will enter the objective ; the greater 

 the angle of the aperture, the more light, and to a certain 

 extent greater resolv^ing power ; there is no evidence, however, 

 for the statement that the resolving of test-diatoms and 

 ISTobert's test-lines is a question of angular aperture, since the 

 l-12th inch object-glass may have its angle of aperture ex- 

 tended to 170 deg., the utmost limit compatible with the 

 reception of rays from any object ; nothing is therefore gained 

 in this respect by a reduction of the focal distance. 



Before entering upon the investigation of test-objects, it 

 would be well for every microscopist to ascertain the true 

 magnifying powers and properties of his own instrument, 

 English, American, and Continental European makers differ 

 in distinguishing their instruments ; each maker has 

 generally his own system. The theoretical power of a micros- 

 cope is usually measured from an arbitrary standard of 10 

 inches ; thus a one inch is said to magnify ten diameters ; 

 a J inch, forty diameters. If the standard is taken at five 

 inches, as it is by some, then the power is but one-half as 

 much. The power of the microscope is that of the objective 

 multiplied by that of the eye-piece ; if the objective magnifies 

 ten diameters, and the eye-piece ten, the result is one hundred 

 diameters. 



In a scientific point of view, and next to Robert's test plate 

 we have a marvellous specimen of art in Herr Mollers, diatom 

 type slide ; this single slide contains over 450 diatoms, the 

 whole are arranged and scientifically classified into four series 

 by Herr Grunow, of Vienna, and within such a compass that 

 the whole can be examined simultaneously with a two inch, 

 objective, and under dark ground illumination they present a 

 singular and beautiful appearance. 



There is issued with the slide a " Systematic Catalogue," 

 which admits of easy reference. Thus, there are nineteen 

 objects belonging to the genus Epithemia the same number of 

 Eunotia, five of Merediones, thirteen of Synedra, &c., &c. In 

 the catalogue they are classed in families, — as Epitliemice 

 IferidioncB, Diatomeas Tabellarice, Surirelece, NitzschicB Amphi- 

 pleurcB, CocconeiclcE, AcTmanthecB, CgnibellecE, Gomphonemece, 

 NamculecE, Isthemice, Biddulpliiacece, JEupodiscece, Melosirem, 

 and Chaeto-cerecB, From this it will be easily seen how to 



