292 Alexander Petrunkevitch 



The proportion between the two media is according to my measure- 

 ments as 4 : 5, /. e.y that an image will occupy four divisions of a 

 scale if water is used as the medium of suspension as against five 

 divisions when glycerin-albumen is used. 



The microscope also must be arranged in a special manner. 

 The diaphragms with the mirror and the Abbe lens must be 

 removed. The instrument is then placed on a high box open to 

 the window, with a long sht occupying the whole space between 

 the legs of the stand. Next a scale in the form of a cross with 

 right angles is drawn on bristol board. Each arm of the cross 

 is two centimeters wide and consists of alternating black squares 

 like those on a checker-board. For the sake of convenience as 

 well as to avoid error, the numbers are written in each white 

 square in roman numerals in one direction and arabic in the 

 other. This cardboard is now placed under the microscope so 

 that the distance between it and the spider's eye is exactly 10 cm. 

 Excessive Hght around the eye is excluded by means of a small 

 diaphragm or a black paper with a small round hole arranged so 

 that the spider's eye hangs directly in the middle of the hole. Of 

 course any objective with small magnifying power may be used. 

 As for myself I either use the a* or the A achromatic system of 

 Zeiss and the compensation ocular 6 with the ocular micrometer- 

 This micrometer is adapted to apochromats but may just as well 

 be used with achromats if one ascertains the size of each division. 

 In my instrument each division of the micrometer with the 

 A objective corresponds to 0.0164 mm., while the correction of 

 the a* lens makes possible a magnification where each division 

 will correspond to o.i mm. The light reflected from the white 

 bristol board on which the scale is made, is sufficient to give a 

 perfect image of the scale in the microscope. 



The scale is so placed that the center of the cross falls exactly 

 on the axis of the spider's eye. I found that the eyes of the hunt- 

 ing spiders are quite round and that the maximum angle of vision 

 is therefore the same in each direction, i. e., the limit of the field 

 of vision in such eyes is a circle representing the circumference 

 of the base of a cone. In order to find the maximum angle in 

 each case, there remains only to read on the image of the scale 



