(i) While viewing the collinearity of pins and images, the eye should be kept at a distance from the pins so that all of them can be seen simultaneously. The collinearity of pins fixed on one side of the glass prism and the images of pins on the other side could also be confirmed by moving the head slight to either side while viewing them. All the pins and images of pins would appear to move together if they are collinear.
(ii) The pins P1, P2, P3 and P4 fixed on the paper may not be exactly perpendicular (or vertical) to the plane of paper. It is therefore desirable to look at the feet of the pins or their images while establishing their collinearity. That is why the position of each pin is marked with pointed tip of the pins on the paper.
