TO DISPLAY OBJECTS MICROSCOPICALLY 419 



is projected by a narrow axial illuminating pencil.' This is a weighty 

 judgment, and should receive full consideration. At the same time 

 the use of wide and solid cones is so full of suggestive results that 

 we must employ them with all possible control by other means of the 

 images they present. This is the more a necessity since Mr. Nelson 

 has been able to obtain the most wonderful results w r ith narrow cones, 

 * true ghosts ' and ' false ghosts,' the presence of ' intercostal markings' 

 in the image of a fly's eye (!), and many complex and false images 

 with the coarser diatoms. But with wide cones he has proved that 

 these false images cannot be produced ; and that when the true image 

 is reached by a wide cone, the image is not altered by any change of 

 focus, but simply fades in and (Kit of focus ' as a daisy under a 

 4-inch objective.' 



Mr. Nelson has photographed all these results, 1 and we have seen 

 them demonstrated. When theory and practice are thus at variance 

 we must pause for further light. 



If it is required to accentuate a known structure, such as the per- 

 forated membrane of a diatom, it can be done by annular illumination, 

 which means the same arrangement as for dark ground, but with a 

 stop insufficiently large to shut out all the light. This method is not 

 to be recommended \vheii a structure is unknown, as it is also liable 

 to give false images. It must be remarked that diatom and other 

 delicate structure, when illuminated with a narrow-angled cone, gives 

 on slight focal alterations a variety of patterns like a kaleidoscope ; 

 with a wide-angled cone a single structure gives a single focus, i.e 

 it goes completely out of focus on focal alteration. When a large- 

 angled and a wide-angled objective are used a change of pattern, only 

 occurs when the structure is fine. This practical observation has its 

 value, and must not be forgotten. 



To properly display objects under a microscope is to a certain ex- 

 tent an art, for it not only demands dexterity in the manipulation 

 of the instrument and its appliances, but it also requires knowledge 

 of what sort of illumination is best suited to the particular object. 

 At this point we think it advisable, especially in the interests of 

 beginners, to clearly point out the best method of commencing 

 microscopic work by centring the condenser and arranging the 

 light for the critical examination of an object. 



1st. Place a powder of about a on the nose-piece, and a B or 

 No. 2 eye-piece in the tube. 



2nd. Use as a source of illumination the light from a paraffin 

 lamp with a ^-inch wick. 



3rd. Place any suitable object on the stage, and, having focussed 

 it with any kind of illumination, centre it to the field of the eye- 

 piece. 



4th. Place a small diaphragm beneath the sub -stage condenser, or 

 close the iris. 



5th. Rack the condenser until the hole in the diaphragm is in 

 focus (in the plane of the object). 



6th. If the hole in the diaphragm should not be central to the 



1 Journ. E. M. S , 1891, p. 90, pi. II. 



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