338 The Microscope. 



refraction or bending, and the optician is thereby enabled to 

 give us objectiv^es that will do better work than he could if the 

 slip and the cover were different in composition from that of 

 the front of the objective. Some of the lenses that form the 

 objective are of flint glass, and in the celebrated apochromatics 

 made b}^ Zeiss, of Germany, some of these component lenses are 

 of fluor spar. But usually the objective is composed of crown 

 and of flint lenses, the flint correcting certain undesirable optical 

 qualities introduced by the crown glass lenses. 



This principle of similarity in composition of slip and cover 

 and front lens is carried still further in the homogeneous 

 immersion objectives of modern times. Here the front of the 

 objective is wetted with a drop of fluid medium whose optical 

 qualities are, as nearly as possible, the same as those of the slip, 

 the cover and the front lens; and if the object is mounted in a 

 proper medium, the entire combination forms one homogeneous 

 whole, through which the light from the microscope mirror 

 passes without suffering ver^^ noticeable optical change until it 

 has entered through the front lens. These are the best object- 

 ives made b^' the modern optician. Water immersions, in which 

 a drop of water is placed between the front lens and the cover 

 glass, are better than dry objectives, that are used without 

 water or any other medium except the air between their front 

 and the cover, because the water drop acts in a manner some- 

 what similar to the drop of homogeneous immersion medium, 

 although not to the same perfect degree. 



MORE ABOUT CEMENTS.— I. 



J. D. BECK. 



INVALUABLE articles have been written on the nature of 

 cements and varnishes for finishing mounts. I feel my 

 incompetence to enter a field among superior microscopists 

 with my suggestions, except on important points which, in my 

 opinion, have been overlooked, as I do not recollect to have 

 read any comments as to the applying of anything on top of the 

 cover glasses of mounts. For objectives of low power, this 

 answers very well, if it is a good, hard and elastic varnish or 

 cement, fixing the cover-glass securely, but when it is desirable 

 to use high-power objectives of short working distance, these 



