722 SU1I31ARY OF CURRENT RESEARCHES RELATING TO 



As to visual and chemical foci, Dr. Kiaer found, by lamplight, no 

 difference either with low or high powers. Lamplight has very few 

 actinic rays, which probably are united at the same plane as the 

 visual ones. 



By sunlight, moreover, it is often difficult to avoid interference 

 effects, whereby the margins of the image are surrounded by dark 

 lines, while by lamplight this inconvenience is avoided. 



Another advantage of lamplight is its not being injurious to the 

 preparation, whereas the warmth of the sun may liquefy and blister 

 the gelatine-glycerine, in which the objects may be mounted. 



Focusing the Image in Photomicrography.* — " Every operator," 

 ■wi-ites Mr. G. E. Davis, " has at one time or another of his experience 

 had great difficulty in satisfying himself of the necessary sharpness 

 of the image on the ground glass. A'eterans of the art are known 

 to have constructed appliances by means of which many of the diffi- 

 culties may be bridged over ; but the tyro is, as a rule, unacquainted 

 with these so-called ' little dodges,' and therefore we pui'pose devoting 

 a little of our space to the description of several methods for getting 

 the exact focus of microscopic objects on the ground glass. 



Dealing with low powers is not so troublesome as with high ones, 

 as there is always sufficient light to enable a tolerably good focus to 

 be obtained ; but with high powers, and consequent loss of light, it 

 requires all the skill at the operator's command to obtain even a 

 passable picture in focusing by means of the ground glass alone. It 

 has been the practice with some to use the finest ground glass obtain- 

 able, and to oil this over with olive oil, whilst others have discarded 

 the use of ground glass as a focusing medium, and have thrown the 

 pictures upon fine Bristol cardboard placed in exactly the same plane 

 subsequently occupied by the sensitive surface of the plate. 



There is no doubt that the oiled ground glass enables the picture 

 to be more accurately focused than when an unprepared surface is 

 employed, but the want of light in the case of high powers is a diffi- 

 culty not dealt with by this method. 



Some years ago, Mr. J. B. Dancer described to us his method, 

 which is as follows: — Draw two lines over the roughened surface of 

 the ground glass from corner to corner, with a writing diamond, and 

 in the centre, where the lines cross, cement a thin cover-glass, three- 

 quarters of an inch in diameter, with balsam and benzol. This pro- 

 duces a transparent cii-cle, and as aids other circles of a similar 

 character may be dotted over the plate in the portion usually occupied 

 by the pictui-e. 



Upon throwing the enlarged image upon a ground glass prepared 

 as above, a little effort will enable the operator to distinguish the 

 details of the picture upon the transparent portion, and in many- 

 cases, without any further aid, an exceedingly sharp focus may be 

 obtained. In many cases, however, it is better to use an auxiliary 

 Microscope to examine this image on the transparent circle. Such 

 an auxiliary Microscope may be easily constructed : a piece of brass 

 tube to hold the A ocular at its upper end, while the lower end is 



* Micr. News. iii. (1883) pp. 233-4 (1 fig.). 



