August 9, 1907] 



SCIENCE 



181 



lished by the Zeiss firm in announcing the 

 new outfit for sale, seemed to be only too true.* 



The ultra-violet light of the cadmium spark 

 being absolutely invisible (it can not even 

 enter the human eye owing to the opacity of 

 the lens to rays of so short a wave-length), 

 it was necessary to devise some system for 

 focussing the objects preparatory to photo- 

 graphing them. For this purpose, Kohler has 

 used a very ingenious " seeker " which con- 

 sists of a simple quartz lens and a fluorescent 

 screen placed over the eyepiece. This screen 

 lights up under the action of the ultra-violet 

 rays. If the objects under the microscope be 

 brought to a focus on this screen the image, 

 when the seeker is removed, will be thrown to 

 a focus on the photographic plate some 30 cm. 

 above. 



Ordinary glass being perfectly opaque for 

 the rays from the cadmium spark, it is, of 

 course, necessary to make of quartz not only 

 the prisms for separating the rays used for 

 photographing with this microscope, but also 

 the collector and collimator lenses, the sub- 

 stage condenser, the slide and cover, the ob- 

 jective and the eyepiece. Even the ordinary 

 glass substage mirror can not be used but must 

 be replaced by a totally-reflecting quartz 

 prism. 



When high power monochromatic objectives 

 are used (and these alone give resolution 

 superior to that of a good visual objective), 

 it is found to be tedious and difiicult to get the 

 object in focus owing to the danger of screw- 

 ing the objective down too far and breaking 

 the cover glass, if not the objective itself. 

 When finally the object is seen, it is found to 

 be impossible to get a sharp focus on the 

 minute details which it is desired to photo- 

 graph, because of the dimness of the image 

 shown by the seeker. Very minute or very 

 delicate objects, such as bacteria and small 

 protozoa often can not be seen at all, and the 

 observer must focus on an air bubble or some 

 chance particle of dirt in the hope that some 



' " With light of considerably different wave- 

 length, more particularly daylight, our Monochro- 



mats cannot ever be used.' 

 M. 170, Jan., 1905, p. 6. 



Carl Zeiss, Circular 



of the objects he seeks may lie in the same 

 plane. Such minute, unstained living cells or 

 the equally small constituent organs of larger 

 cells are, however, of most interest for study 

 with the ultra-violet microscope, not only be- 

 cause of the superior resolving power of the 

 new lenses, but also because, owing to the 

 opacity of many parts of the cell to ultra- 

 violet light, the photographs show such living 

 cells as if they had been fixed and stained, giv- 

 ing a welcome proof of the reality of the 

 structures observed in the cells after killing 

 and staining. 



While trying to use one of the new micro- 

 scopes" in April, 1906, on such objects, we hit 

 upon a new and in our opinion much better 

 method of focusing. 



Instead of employing a single pair of elec- 

 trode holders as planned by the makers (Fig. 

 1), we use a double pair of holders (four in 

 all) arranged so that the cadmium electrodes 

 can be instantly swung out and replaced by a 

 pair of magnesiiim electrodes by means of the 

 handles shown in Fig. 2. The cadmium elec- 

 trode holders are longer than those for the 

 magnesium for a purpose to be explained 

 later. There is an automatic stop on the 

 lower pair of holders to insure the spark-gap 

 falling in the axis of the collimator lens. 



We were led to devise such a swing-out elec- 

 trode changer by discovering that the mono- 

 chromatic lenses, through giving only badly 

 blurred and colored images with ordinary 

 light, did give very good images that could 

 he focused sharply even to the finest detail, 

 providing strictly monochromatic visihle light 

 were used. The spark spectrum of magnesium 

 shows a well isolated line in the blue that 

 proved to be very well adapted for making ex- 

 ploratory "observations and for focusing. The 

 wave-length of this line is 448 ju/*. It is near 

 the line O (431 jj,fi) of the solar spectrum. 



In using the ultra-violet microscope by our 

 method the object is first found and centered 

 with a low power visual lens, using the 

 magnesium blue light Then the high-power 



' Kindly loaned by Mr. H. G. Kribs, pending the 

 arrival of the highest power objective ordered 

 from Germany. 



