86 THE MICROSCOPE. 



aperture will admit of ; x hence three different fronts 

 would give a great range of aperture with a correspond- 

 ing working distance, which is practically what is 

 sought by having objectives constructed of the threa 

 different foci, |, T V, and -5^-." 



" There can be no doubt," adds the writer, " English 

 Mechanic," "that the development of the homogeneous- 

 immersion system, is the problem of the future as 

 regards attaining the limit of resolution with the micro- 

 scope." But to ensure the fullest advantages the system 

 is capable of, a fluid is wanted which will meet all its 

 requirements. Professor Abbe, Mr. Stephenson and others 

 have experimented on various substances, and the con- 

 clusion come to is that the essential oils of cedar wood, 

 or fennel, which so nearly correspond to glass in their 

 refractive and dispersive power, will be found to afford 

 the best results. Oil of aniseed, chloral hydrate and 

 glycerine, various turpentines, and lead solutions have 

 been tried and ultimately abandoned. One precaution 

 is required with regard to all essential oils, they can only 

 be used with objectives having their fronts specially 

 cemented. Very fine objectives constructed on the 

 homogeneous immersion system by Zeiss, and requiring 

 no adjusting collar, may be had of Baker, Holborn, the 

 London agent of this optician. 



THE CONSTRUCTION OF THE MICROSCOPE. THE MICRO- 

 SCOPE STAND. 



The Principal Forms of Microscopes. Having duly 

 considered the essential parts of the compound achro- 

 matic microscope, I shall proceed to offer a few re- 

 marks on typical forms of English manufacture. I 

 must not attempt a critical examination of the very 

 numerous stands known to practical microscopists. 

 This would be impossible in a limited space. Neither 

 shall I attempt to institute invidious comparisons, as, 

 in my opinion, most forms of instruments brought 

 to notice possess some feature of a useful and praise- 



(1) Powell and Lealand have also constructed a V and 53 on the homo 

 gcncons system, but the apertures are nots/> high as in the $ just mentioned 



