1 2 Transactions. 



place; one devised by Dr. J. J. Woodward, which recommends 

 itself from its simplicity and cheapness ; the other by Dr. James 

 Edmunds, described last year before the Royal Society, which, 

 from the accounts we hear of it, is destined to become a 

 valuable accessory. 



Dr. Woodward's apparatus is figured on page ii (Fig. i, 

 reduced from the M. M. J.), and may be thus briefly de- 

 scribed : Just beneath the slide F, and connected with it by 

 oil of cloves or other highly refractive medium, is a truncated 

 right-angled glass prism, resting upon a similar brass prism 

 supported on the sub-stage. D is a shutter, parallel to the 

 side of the prism as shown, with a small hole E about like 

 a large pin hole. This side of the prism is covered with black 

 paper with a corresponding pin hole. 



A balsam-mounted object, viewed with an immersion lens 

 of sufificient angular aperture, can thus be seen with sunlight 

 from the plane mirror at an angle of 45° from the axis. It is 

 only by some such plan as this that we are able to obtain light 

 of this obliquity in examining balsam-mounted objects. 



We now pass to the more costly appliance of Mr. Edmunds. 

 This consists of a paraboloid lens, with the front cut off flat 

 and polished. The extent to which the apex is cut off depends 

 upon the conditions of use, and may be one-twelfth of an inch 

 below its internal focus, or barely one-fiftieth of an inch be- 

 low. 



This lens is used beneath the slide, in fluid contact with it, 

 and the thickness of the slide determines the place of cutting; 

 the object being to secure a lens which shall have its focus for 

 parallel rays just at the upper surface of the slide when in 

 fluid contact. Diaphrams and shutters of any kind may be 

 arranged beneath the lens, and any rays can thus be selected 

 for use at pleasure. Such a lens has peculiar properties, 

 which we should observe in order to understand its action and 

 advantages. Excluding the more central rays, light entering 

 from below will not pass out again into air above, but suffer 

 total internal reflection. This is not true of the Wenham 

 paraboloid in common use, and this very fact makes the new 

 lens the more valuable for use with high powers for resolu- 

 tions. 



Place a slide upon the stage and interpose a drop of gly- 



