ARRANGEMENT TOR TRANSPARENT OBJECTS. 157 



by daylight is recommended never to omit ascertaining whether 

 the view which he may obtain of them is in any degree thus 

 marred by incident light. 



118. Although the illumination afforded by the Mirror alone is 

 quite adequate for a very large proportion of the purposes for which 

 the Microscope may be profitably employed (nothing else having 

 been used by many of those who have made most valuable contri- 

 butions to Science by means of this instrument), yet, when high 

 magnifying powers are employed, and sometimes even when but a 

 very moderate amplification is needed, great advantage is gained 

 from the use of a Condenser. The form which has been described 

 under the name of the Webster Condenser (§ 80) answers so well 

 for most purposes, and may in addition be so easily converted into 

 a Black -Ground Illuminator, that the working Microscopist will 

 find it convenient to keep it always in place ; substituting an 

 Achromatic Condenser of greater power (§ 79) only when specially 

 needed. 'Special care is needed in the use of this last, both as to 

 the coincidence of its optic axis with that of the Microscope 

 itself, and as to its focal distance from the object. The centering 

 may be most readily accomplished by so adjusting the distance of 

 the Condenser from the Stage (by the rack-and-pinion action, or the 

 sliding movement, with which it is always provided), that a sharp 

 circle of light shall be thrown on any semi-transparent medium 

 laid upon it ; then, on this being viewed through the Microscope 

 with an Objective of sufficiently low power to take in the whole of 

 it, if this circle be not found to be concentric with the field of 

 view, the axis of the Condenser must be altered by means of the 

 milled -head tangent-screws with which it is provided. The focal 

 adjustment, on the other hand, must be made under the Objective 

 which is to be employed in the examination of the object, by 

 turning the Mirror in such a manner as to throw upon the visual 

 image of the object (previously brought into the focus of the Micro- 

 scope) an image of a chimney or a window-bar, if daylight be 

 employed, or of the top, bottom, or edge of the lamp-flame, if 

 lamp-light be in use ; the focus of the condenser should then be so 

 adjusted as to render the view of this as distinct as possible; and 

 the direction of the Mirror should then be sufficiently changed to 

 displace the image, and to substitute for it the clearest light that 

 can be obtained. It will generally be found, however, that although 

 such an exact focussing gives the most perfect results by Daylight, 

 yet that by Lamp-light the best illumination is obtained when the 

 Condenser is removed to a somewhat greater distance from the 

 object than that at which it gives a distinct image of the lamp. In 

 every case, indeed, in which it is desired to ascertain the effect of 

 variety in the method of illumination, the effects of alterations in 

 the distance of the condenser from the object should be tried ; as 

 it will often happen that delicate markings become visible when 

 the condenser is a little out of focus, which cannot be distinguished 



