=1. Methods of Judging Distance by Eye.=--Distances may be judged by eye in the following ways: (_a_) By measuring the intervening ground with the eye in terms of some familiar unit, such as 100 yards; (_b_) by the apparent size of the object, if its size is known, or, in other words, by visual angles; (_c_) by the visibility of the object as affected by light, atmospheric effect, background, etc. =2. Rules for Judging Distance by Eye.=--(i) The varying nature of the ground and its features, together with different conditions of light and atmosphere, affect general impressions of distance as measured by the eye by making objects look smaller or larger, and so making them seem nearer or further away from an observer than they actually are. (ii) =Conditions which appear to Increase Distance.=--Objects will appear smaller, and therefore farther away than they are, under the following conditions: (_a_) When objects are of the same colour or a colour which harmonizes with their background. (_b_) On broken ground. (_c_) When seen across a valley or undulating ground. (_d_) In avenues, long streets, or ravines. (_e_) When in shadow. (_f_) When viewed in mist or failing light. (_g_) When heat is rising from the ground. (_h_) When seen near any other object which makes it appear smaller than it is by the effect of comparison. (_i_) When only partially seen, as in the case of troops firing from cover. (_j_) Troops kneeling or lying in the open seem farther away than when standing. (iii) =Conditions which appear to Decrease Distance.=--Objects will appear larger, and therefore nearer than they are, under the following conditions: (_a_) When both object and background are of different or contrasting colours. (_b_) When the sun is behind the observer. (_c_) In bright light or clear atmosphere. (_d_) When the intervening ground is level or covered with snow. (_e_) When seen across water or a deep chasm. (_f_) When looking upwards or downwards. (_g_) When the object is large, or when seen near any other object which makes it appear larger than it is by effect of comparison. (iv) =A General Rule.=--A good general rule to remember for practical purposes is that _when a target is indistinct distance is apt to be over-estimated, and that when a target is distinct distance is apt to be under-estimated_. This general rule roughly sums up the effects and the various factors mentioned in paras. (ii) and (iii), which respectively tend to render targets less or more distinct.