Context :

Aims : We present an exploration of weak lensing by large-scale structure in the linear regime , using the third-year ( T0003 ) CFHTLS Wide data release .
Our results place tight constraints on the scaling of the amplitude of the matter power spectrum \sigma _ { 8 } with the matter density \Omega _ { m } .

Methods : Spanning 57 square degrees to i ^ { \prime } _ { AB } = 24.5 over three independent fields , the unprecedented contiguous area of this survey permits high signal-to-noise measurements of two-point shear statistics from 1 arcmin to 4 degrees .
Understanding systematic errors in our analysis is vital in interpreting the results .
We therefore demonstrate the percent-level accuracy of our method using STEP simulations , an E/B-mode decomposition of the data , and the star-galaxy cross correlation function .
We also present a thorough analysis of the galaxy redshift distribution using redshift data from the CFHTLS T0003 Deep fields that probe the same spatial regions as the Wide fields .

Results : We find \sigma _ { 8 } ( \Omega _ { m } / 0.25 ) ^ { 0.64 } = 0.785 \pm 0.043 using the aperture-mass statistic for the full range of angular scales for an assumed flat cosmology , in excellent agreement with WMAP3 constraints .
The largest physical scale probed by our analysis is 85 Mpc , assuming a mean redshift of lenses of 0.5 and a \Lambda CDM cosmology .
This allows for the first time to constrain cosmology using only cosmic shear measurements in the linear regime .
Using only angular scales \theta > 85 arcmin , we find \sigma _ { 8 } ( \Omega _ { m } / 0.25 ) _ { { lin } } ^ { 0.53 } = 0.837 \pm 0.084 , which agree with the results from our full analysis .
Combining our results with data from WMAP3 , we find \Omega _ { m } = 0.248 \pm 0.019 and \sigma _ { 8 } = 0.771 \pm 0.029 .

Conclusions :