A SIMPLE CONDENSEK 313 



The iris diaphragm is for general purposes more convenient than 

 the usual circular plate, Imt it has the drawback of being incapable 

 of setting to any exact size. A delicate point in an image, caught 

 with a certain-sized diaphragm, is not regained with ease and cer- 

 tainty with the iris, 1 and may involve much patience and labour; 

 but a well-made large plate of graduated diaphragms will wholly 

 remove this difficulty. Moreover, for testing object -glasses it is 

 supremely important that a metal diaphragm be used, so that the 

 conditions of illumination may be readily and accurately reproduced. 



It may be of service to those who are unable or indisposed to 

 spend considerable sums upon condensers to state that an excellent 

 achromatic condenser can be made by placing a Zeiss aplanatische 

 Lupeii ' on Steinheil's formula in the sub-stage.- This plan has 

 been adopted in one of Reichert's stands, as we have seen. 

 These are made in two different powers, viz. 1 inch and H inch, and 

 we can fully testify to their being the most useful hand-lenses for 

 ordinary work that can be employed. (Treat credit is due to Dr. 

 Zeiss for bringing out such excellent achromatic lenses at so low a 

 price, and so meeting a want long and generally ielt. Excellent 

 forms of triplet lenses answering a similar purpose are made by 

 Bausch and Lomb after the calculations of Professor Hastings, and 

 most leading makers. Continental and English, make similar magni- 

 fiers to those of Zeiss. An achromatic loup of this kind is almost an 

 indispensable accompaniment of a microscopic outfit, and. if a tube to 

 receive it be arranged in the sub-stage, these lenses make really ex- 

 cellent condensers for low powers. It need not have a centring sub- 

 stage, but only a central fitting. It is not of 'course qualified to 

 supplant the condenser of larger and more perfect instruments, but it 

 is capable of raising students' and other simple microscopes to a much 

 higher level. 



Without a condenser the microscope is either (by construction) 

 not a scientific instrument, or it is an instrument unscientifically 

 used. It becomes a mere ' magnifying glass.' It is the adaptation 

 for and use of a condenser though as simple as a hemispherical lens 

 fitted into a stage plate that raises it to a microscope. 



We have already referred to the nature of the mechanical 

 arrangements needful for the condenser in a general way (Chapter 

 III., pp. 185-190) ; we may add here that the simplest form of sub- 

 stage being a tube fixed centrally in the optic axis of the microscope. 

 the simplest form of condenser-mount will be a tube sliding into 

 this. It must not screw, it must push, and there should be a little 

 below the back lens a shoulder to hold the diaphragms. stop>. glasses, 

 etc. Centring gear is not necessary with students' and elemental v 

 microscopes. The slight displacements due to varying centres of 



1 It will be urged that apertures can be exactly reproduced with the iris in 

 photographic lenses ; why cannot they, therefore, in the case of the microscope ? 

 The answer is 1 1) that with wide-angled condensers a very slight difference in the 

 aperture makes a very great difference in the angle ; a similar difference would be 

 inappreciable in the case of a photographic lens, i 2) It is in small apertures such 

 as are seldom used in photographic lenses where the difficulty arises in the case of 

 the microscope. (3) It is in the small apertures that the iris fails to respond to 

 the movement of the lever. 



2 Joiirn. (Jm-kett 3//r. ('///, vol. iv. ser. ii.p. 77 (1889), on Zeiss's loup. E.M. Nelson. 



