POWELL AND LE ALAND'S CONDENSER 301 



by Messrs. Powell and Lealand. We have used this instrument for 

 thirty-five years on every variety of subject, and we do not hesitate 

 to affirm that for general and ordinary critical work it is still un- 

 surpassed. Fig. 240 illustrates this apparatus. The optical com- 

 bination is a 1th of an inch power, and it is therefore more suitable 

 for objectives from a Jth of an inch and upwards; but by removing 

 the front lens it may be used with objectives as low as one inch. 



Having given to this condenser so high a place amongst even 

 those of our immediate times, it may be well to specify what the 

 requirements arc which a condenser employed in critical work with 

 high powers should meet. It is needful that we should be able 

 (1) to obtain at will the largest ' solid ' cone of light devoid of 

 spherical aberration. 1 Directly spherical aberration makes itself 

 apparent the condenser fails ; that is, when, on account of under- 

 correction, the central rays are brojught to a longer focus than the 

 marginal rays, or when, because of over-correction, the marginal 

 rays have a longer focus than the central. 



But (2) it is also an absolute essential that if a condenser is to 

 be of practical service it must have a 

 working distance sufficiently large to 

 enable it to be focussed through 

 ordinary slips. It would be an 

 advantage if all objects mounted 

 for critical high-power work were 

 mounted on slips of a fixed gauge, say 

 06- inch, which would be ' medium,' 

 05 inch being accounted ' thin,' and 

 07 inch ' thick.' 



It is plain, however, that to FIG. 240.-Powell and Lealand's 

 combine a large aperture with a condenser, 



great working distance the skill of 



the optician is fully taxed, for this can only be accomplished (a) by 

 keeping the diameter of the lenses just large enough to transmit 

 rays of the required angle and no more ; (b) by working the convex 

 lenses to their edge ; (c) by making the flint lenses as thin as 

 possible. 



Xow it is due to the eminent firm whose condenser we have 

 been considering with such appreciation to say that the condenser 

 referred to (d) did, when it was first devised and for many years 

 after, transmit the largest ' solid ' cone free from spherical aberra- 

 tion ; (e) that it has the greatest working distance ; (/ ) that its 

 chromatic aberrations are perfectly balanced. In the possession of 

 these three essential qualities it stood unrivalled for upwards of 

 thirty years. 



The removal of the front lens of this condenser, which may be 

 readily unscrewed, reduces it in power and angle, and therefore 

 makes it suitable for objectives of lower power. This, however, is 

 rather an adaptation involving compromise than an ideal condenser 



1 This is one of the many expressions which are inevitable to the practical use of 

 apparatus; it is simply convenient, and means a full cone of light a cone with none 

 of its rays stopped out. 



