OF THE 



Achromatic Doublets and Triplet*. Magnifiers, 

 composed of two or more lenses, are to be had of two very 

 distinct kinds. The lenses may either be simply united in one 

 frame, without any special adaptation to each other, or the in- 

 strument may consist of two or more achromatic lenses com- 

 bined together in a fixed and accurately determined relation. 

 Examples of the former are found in the ordinary two and three 

 lens magnifiers we have just described ; the latter are not so 

 common, since they are somewhat expensive when well made. 

 They are known as achromatic doublets and triplets, and one 

 maker in this country, Mr. Tolles, of Boston, has long been 

 noted for the excellence of the simple achromatic microscopes 

 of this class made by him. 



The advantages possessed by lenses of this kind are the larger 

 field of view which they give, thus enabling us to obtain a clear 

 view of objects of considerable size ; and the wonderfully in- 

 creased sharpness of definition which they afford, owing to their 

 wide angular aperture. They are usually mounted in the same 

 style as the Coddington (Fig. 9), but are readily distinguished 

 from the latter by the absence of any diaphragm. The lenses 

 of which these magnifiers are composed are so constructed and 

 combined that the field is perfectly flat and well defined in all 

 its parts, so that a diaphragm is not necessary. Mere common 

 lenses, put together so as to resemble these achromatic doublets, 

 and without a diaphragm, would give such a misty view of ob- 

 jects that they would be at once condemned. 



With the exception of the low and medium-power achromatic 

 objectives used with the compound microscope, these doublets 

 and triplets, when well made, are altogether the most satisfac- 

 tory simple microscopes in use, and several firms now make a 

 specialty of their manufacture. Amongst others we may name 

 the Bausch & Lomb Optical Company, Messrs. E. & J. Beck, 

 Browning, whose magnifiers of this kind are known as Platy- 

 scopic lenses, and Steinheil, of Munich. 



To some it may appear that we have devoted more space to 

 the simple microscope than its importance demands. Our 

 excuse is that simple microscopes, of the different kinds we 

 have just described, are not in such general use as they ought 

 to be. This, however, seems to be the case even in France, the 



