The pressure a liquid exerts against the sides and bottom of a container depends on the density and the depth of the liquid. If atmospheric pressure is neglected, liquid pressure against the bottom is twice as great at twice the depth; at three times the depth, the liquid pressure is threefold; etc. Or, if the liquid is two or three times as dense, the liquid pressure is correspondingly two or three times as great for any given depth. Liquids are practically incompressible – that is, their volume can hardly be changed by pressure (water volume decreases by only 50 millionths of its original volume for each atmospheric increase in pressure). Thus, except for small changes produced by temperature, the density of a particular liquid is practically the same at all depths. Atmospheric pressure pressing on the surface of a liquid must be taken into account when trying to discover the total pressure acting on a liquid. The total pressure of a liquid, then, is ρgh plus the pressure of the atmosphere. When this distinction is important, the term total pressure is used. Otherwise, discussions of liquid pressure refer to pressure without regard to the normally ever-present atmospheric pressure. The pressure does not depend on the amount of liquid present. Volume is not the important factor – depth is. The average water pressure acting against a dam depends on the average depth of the water and not on the volume of water held back.
Accommodative insufficiency (AI) involves the inability of the eye to focus properly on an object. Accommodation is the adjustment of the curvature of the lens to focus on objects near and far. In this condition, amplitude of accommodation of a person is lesser compared to physiological limits for his age. AI is generally considered separate from presbyopia, but mechanically both conditions represent a difficulty engaging the near vision system (accommodation) to see near objects clearly. Presbyopia is physiological insufficiency of accommodation due to age related changes in lens (decreased elasticity and increased hardness) and ciliary muscle power.AI is commonly present in people with convergence insufficiency. Categories
Accommodative insufficiency is further categorised into different categories. Ill-sustained accommodation
Ill-sustained accommodation is a type of accommodative insufficiency in which, range of accommodation will be normal, but after excessive near work accommodative power will decrease. Paralysis of accommodation
In paralysis of accommodation, amplitude of accommodation is either markedly reduced or completely absent (cycloplegia). It may occur due to ciliary muscle paralysis or oculomotor nerve paralysis. Parasypatholytic drugs like atropine will also cause paralysis of accommodation. Unequal accommodation
If there is amplitude of accommodation between the eyes differ 0.5 dioptre or more, it is considered as unequal. Organic diseases, head trauma or functional amblyopia may be responsible for unequal accommodation. Causes
Premature sclerosis of lens or ciliary muscle weaknesses due to systemic or local cases may cause accommodative insufficiency. Systemic causes of ciliary muscle weakness include diabetes, pregnancy, stress, malnutrition etc. Open angle glaucoma, Iridocyclitis etc. are known local causes.