338 



MICROSCOPE. 



an equal magnifying power, and the aperture 

 would admit of great extension without a pro- 

 portional increase in the spherical and chro- 

 matic aberrations. This suggestion has been 

 carried into practice with complete success as 

 regards the performance of lenses executed on 

 this plan ; but the difficulties of various kinds 

 in the way of their execution are such as to 

 render them very expensive ; and as they are 

 not superior to the combination now to be de- 

 scribed, they have latterly been quite super- 

 seded by it. 



This combination was first proposed by Dr, 

 Wollaston, and is known as his doublet. It 

 consists of two plano-convex lenses, whose 

 focal lengths are in the proportion of one to 

 three, or nearly so, having their convex sides 

 directed towards the eye, and the lens of 

 shortest focal length nearest the object. In 

 Dr. Wollaston 's original combination no stop 

 was interposed, and the distance between the 

 lenses was left to be determined by experiment 

 in each case. A great improvement was subse- 

 quently made, however, by the introduction 

 of a stop between the lenses, and by the divi- 

 sion of the power of the smaller lens between 

 two; this is due to Mr. Holland.* By these 

 means a combination may be produced, in 

 which the errors are made to correct each other 

 so nearly that all the advantages of a wide 

 aperture with a very short focus may be gained. 

 The general nature of the performance of a 

 doublet or triplet may be understood from the 

 adjoining figure, (fig AGO,} in which L L' is 



Fig. 160. 



u 



Diagram of the passage of rays through a doublet, 



the object, P a portion of the pupil, and D D 

 the stop. The pencils of light from the two 

 extremities, L L', of the object cross each 

 other in the stop, and consequently pass 

 through the two lenses on the opposite sides 



' Trans, of Soc. of Aits, vol. xlix. 



of the axis O P ; so that each becomes af- 

 fected by opposite errors, which to a certain 

 extent balance and correct one another. To 

 take the pencil L, for instance, which enters 

 the eye at R B, R B; it is bent to the right 

 at the first lens, and to the left at the second ; 

 and as each refraction alters the direction of 

 the blue rays more than of the red, and more- 

 over, as the blue rays fall nearer the margin 

 of the second lens, where the increased power 

 of the refraction, consequent upon the distance 

 from the centre, compensates in some degree 

 for the greater focal length of the second lens, 

 the blue and red rays will emerge very nearly 

 parallel, and are therefore colourless to the eye. 

 At the same time the spherical aberration has 

 been diminished by the circumstance that the 

 side of the pencil, which passes one lens 

 nearest the axis, passes the other nearest the 

 margin. This explanation applies only, how- 

 ever, to the pencils near the extremities of the 

 object. The central pencil, it is obvious, will 

 pass through the same relative portions of the 

 two lenses, and only an imperfect correction 

 will therefore take place, and of those issuing 

 from the intermediate points the amount of 

 correction will vary with their proximity to the 

 centre or to the circumference. Hence a dou- 

 blet is not a perfect magnifier; but it is very 

 much superior to a single lens, and may be so 

 constructed as to show many of the usual test- 

 objects, especially those in which a moderate 

 amount of penetration is sufficient, provided 

 the definition be good, in a very beautiful 

 manner. Its angle of aperture, however, by 

 which is meant the angle of the apex of the 

 conical pencils of rays admitted by it, cannot 

 be advantageously increased much beyond 40 

 or 45. But when the smaller lens is replaced 

 by a combination of two others, so as to form 

 a triplet, their joint aberration is so much less 



Fig. 161. 



Diagram to illustrate angle of aperture. 



A, lens with small opening, admitting only pencils 

 of rays diverging at an angle of 15; 15, lens 

 with large opening, admitting pencils of 50. 



that it is more counterbalanced by the third 

 lens placed above the stop. In this manner 

 the transmission of a still larger angular pencil, 

 even to 65, is rendered compatible with 

 distinctness; and great penetrating power is 

 thus combined with perfect definition, as 

 well as with brilliancy of illumination. For 

 the purposes of anatomical investigation, as 

 we shall hereafter state, we consider good 

 doublets and triplets, where circumstances 

 admit of their employment, superior to any 

 other kind of magnifying instrument. The 

 principal disadvantages which the use of them 

 involves are the close proximity to the object 

 required by their very short focus when a high 



