24 SELECTION AND USE 



sive when well made. They are known as doublets and triplets,' 

 and one maker in this country, Mr. Tolles, of Boston, has 

 become famous for the excellence of the simple achromatic 

 microscopes of this class, made by him. These doublets and 

 triplets are altogether the most satisfactory simple microscopes 

 in use, and several firms make a specialty of their manufacture. 

 In addition to Mr. Tolles, the Bausch & Lomb Optical Com- 

 pany, under the direction of Mr. Gundlach, make a very ex- 

 cellent lens of this kind, and in England, Mr. Browning makes 

 a very excellent achromatic magnifier under the name of the 

 Platyscopic Lens. 



Where two or more simple lenses are used together (without 

 being combined so as to form a compound microscope) the 

 power of the combination is always equal to the sum of the 

 powers of the separate lenses. Thus if we have a lens of half 

 an inch focus and one of one inch focus, one magnifying ten and 

 the other twenty diameters, the resulting power is thirty and 

 and not two hundred times. In the compound microscope, on 

 the other hand, the combination of an objective magnifying 

 twenty diameters with an eyepiece magnifying ten diameters, 

 gives a magnifying power of two hundred diameters. 



Two or more lenses, properly adapted to each other and used 

 together, give results greatly superior to anything that can be ob- 

 tained from a single lens, at least so far as clearness and accuracy 

 of definition is concerned. But when used as a working or dis- 

 secting microscope, they are open to the objection that the dis- 

 tance at which they must be placed from the object is very small, 

 and hence it is frequently inconvenient to use them for work- 

 ing upon objects. Thus if we have a plano-convex lens of a 

 quarter of an inch focus, and one of three quarters of an inch 

 focus, and place them at a distance of a sixteenth of an inch from 

 each other, we will have a very good magnifier which will 

 enlarge objects about thirty-five to forty times, but we must 

 place it at but a very short distance from the object. If we 

 separate the lenses a little, the definition will be improved, but 

 the working distance, as it is called, will be diminished. Those 

 who have studied optics are quite familiar with these facts, but 

 the ordinary reader does not always think of them, and yet they, 



