10 BRITISH BUTTERFLIES. 



to the lack of contrast obtainable in the illumination, and particularly 

 to the very great difficulty in focussing accurately when the ground 

 glass image was so faintly lighted. Fortunately I was able to avail 

 myself of an electric light installation, and found a -25 Amp. Nernz 

 lamp admirable in every way, as it gave a 25 to 30 candle-power 

 light, and this, with a single condenser on one side of the object and a 

 mirror reflector on the other to reduce the blackness of the shadows it 

 made, appeared as nearly ideal as I could imagine. I have not tried 

 incandescent gas myself, but I understand from friends who use it 

 that it gives equally good results. Still I should imagine that the 

 heat trouble would obtrude itself, especially if the light were placed 

 near to the object, whereas, with the Nernz lamp, the light can be 

 approached to within 3in. of the ovum being operated on, without any 

 untoward results. The great points to bear in mind are to get a 

 brilliant illumination of the object from one side, and a somewhat less 

 powerful lighting on the other, so as to show up the structure and 

 rotundity of the ovum by contrast. The final critical focussing is best 

 accomplished with the aid of a focussing magnifier held against the 

 ground glass focussing screen, and fixing the attention on some 

 brightly illuminated spot on the surface of one of the ova to be 

 photographed. 



After focussing and placing the sensitive plate in position, it is 

 only necessary to cover the lens with a piece of black card while 

 drawing out the flap of the dark slide, as any small amount of indirect 

 side light entering the lens is quite negligible, and the card is more 

 conveniently manipulated than a lens cap. The exposure necessary 

 will naturally vary with the brilliancy of the illumination and the 

 colour of the ova, but there is a large amount of latitude permissible 

 with most of the dry plates on the market. I use the most rapid 

 isochromatic plates I can obtain, always backed to minimise halation, 

 and find that, with the 3in. microscopic lens and a magnification of 

 twenty diameters, an exposure of two and a half to four minutes, 

 according to the colour of the ova, is about right. With the 5in. 

 Rapid Rectilinear, stop f. 16, magnification ten diameters, and similar 

 illumination, 1 should give from four to seven minutes, but these 

 exposures might be doubled without detriment to the resulting 

 negative by a slight addition of bromide of potassium to the 

 developing solution. 



For holding ova in position while being photographed I use 

 ordinary glass slips 3in. xlin., such as are used for mounting micro- 

 scopic slides. Ova in situ on bark, portion of leaves, paper, etc., are 

 easily fixed with a small dab of adhesive material, and, where needed, 

 a background of suitably toned paper can be gummed on the glass 

 slip first, and then the support for the ova fixed upon it. When the 

 ova are loose, and particularly if it be desired to retain them 

 uninjured for subsequent hatching, 1 find a most suitable method of 

 mounting to be as follows: — Cutout a small square of gummed paper, 

 say |in. each way, and punch out a circular hole in the centre Jin. in 

 diameter. An ordinary cork boring drill does this admirably. Then 

 cut another smaller square of paper, tinted to suit the ova in question, 

 and gummed on one side. This should be a little Larger than the 

 hole alluded to above, say ^.in. square. Place the small square, 

 gummed side iijnranls, in the centre of the glass slip, moisten the gum 



