INTRODUCTION. XXV11 



Owing to the now almost universal use of the standard 

 screw of the Royal Microscopical Society of London, objec- 

 tives of one maker can be attached to the instrument of 

 another. A point of some importance to English purchasers 

 of continental objectives is however this : the customary 

 length of the tube of the microscope on the Continent is 150 

 to 170 millimetres (six to seven inches), and the objectives- 

 are constructed to suit this length. If the tube exceeds this 

 length, it should be stated in ordering the objectives, that 

 they may be modified to suit. This is especially needed in 

 ordering objectives for homogeneous immersion. All the 

 microscope stands mentioned above have tubes of continental 

 length, and mostly of continental size. 



Besides the compound microscope to which we have 

 hitherto exclusively referred, a simple, or so-called prepara- 

 tion or dissecting, microscope is also more or less necessary. 

 For all the purposes of this book, and, indeed, for most 

 botanical purposes, whether in anatomy or morphology, a- 

 dissecting microscope of very simple construction is all that 

 is needed, and two or three are illustrated on pp. 206-8. 

 Most instruments for this purpose are unnecessarily complex 

 and expensive. Some, for instance, are constructed to 

 magnify up to 60, 80, or even 100 diameters ; if such magni- 

 fying power is needed, the low power of the compound micro- 

 scope will do equally well, dissecting being done upon the 

 stage, but the arms being carefully supported. The student 

 can, indeed, entirely dispense with a dissecting microscope, 

 and dissect upon the stage of his larger instrument by the 

 aid of his lowest power ; but as the image of the object is 

 inverted, and any movements he may make are likewise re- 

 versed, he would probably be at first somewhat perplexed. 

 Practice will overcome this difficulty ; or it can be cleared 

 away at once by purchasing an " erector " for insertion in 

 the draw-tube, costing usually IOs. or 10s. 6d. It is desirable 

 likewise to have a low-power objective, e.g., 2-inch or H-inch, 

 though dissection with the 1-inch is perfectly simple. The 

 lowest power eye-piece should be used. Dissection under 

 the compound microscope has, with very small objects, this 

 further advantage, that there is no chance of losing them in 



