96 NOTES ON THE FUTURE OF THE MICROSCOPE : 



been reserved for these instruments. Unfortunately the new flints, 

 the telescope flint, boro-silicate flint, borate flint, etc., have small 

 dispersive power. The lens curvatures have to be exaggerated, the 

 zonal aberrations become disturbing, their correction is troublesome, 

 the objectives are difficult to construct, and, in spite of their very- 

 real superiority over the ordinary achromatic objectives, the price of 

 the apochromatc sometimes makes the buyer hesitate. 



In our opinion, the progress of the microscope, as to easy and 

 compact correction of aberration, will depend much m,ore upon the 

 work of the glass-maker than upon the calculations of the optician. 



We can now form a clearer opinion concerning the interest which 

 extreme magnifications of 5,U00, 10,000 diameters and more can 

 present. On Abbe's theory M. von Rohr has fixed the smallest dis- 

 tance that an objective of aperture 1.40 can resolve at 0.00015 mm. 

 The eye can separate about 1 in^h, say 0.1 at a distance of 33 mm. 

 An enlargement of 700 diams. enables us to see all the details of an 

 object. A more powerful eye-piece only enlarges the image without 

 bringing out any further detail. The image which the observer 

 examines may be less perfect than the normal view; on the other 

 hand, the eye is fatigued by being strained to its maximum effort. 

 For this reason, one has gone up to enlargements of 2,000 and 3,000 

 diameters. This latter magnification is excessive, however, and we 

 have never seen it applied for any useful purpose in microscopy. In 

 microphotography, on the other hand, it is sometimes serviceable to 

 magnify 10,000 times and to use even higher powers — for instance, 

 when the image is to be' exhibited in the lecture theatre, or when 

 one wishes to touch up a proof or to put references on it. 



Photomicrography . — So far we have presumed working in ordinary 

 light. As the photographic plate is sensitive to ultra-violet radiations, 

 we can in photomicrography obtain higher definition by diminishing 

 the ^- One difficulty creeps up at once, however: most of the 

 optical materials are opaque to ultra-violet radiations. Rohr built up 

 the whole optical system out of fused quartz ; there was no correction 

 for chromatism, and illumination was effected b}" one of the aluminium 

 radiations. The index of quartz for D rays is n = 1.54; for the ray 

 AIs2, the index rises to 1.69. The immersion liquid is glycerin; one 

 is, in many cases, restricted by the opacity of the preparation itself. 

 In these respects the limit seems to have ah^eady been attained, or 

 nearly been attained. 



Condensers. — As regards condensers, the constructors may be said 

 to have preceded the microscopist. For delicate researches, non- 

 corrected condensers are frequently used; yet they should be achro- 

 matic. The theory of Abbe assumes that the object is placed in the 

 image of the luminous source. That is, with ordinary condensers, 

 obviously possible only for one single radiation, and one point of the 

 field. A bad illumination is so disastrous that, even at the present 

 hour, many investigators are by no means convinced of the superiority 

 of the apochromatic instrument. One cannot tell them often enough 

 that this defect is solely due to the insufficiency of their condenser 

 and to the poor choice of a luminous source. Achromatic, and even 

 apochromatic condensers are in existence, and a deplorable misjudg- 

 ment alone has prevented their general use. 



